Pypsa

This is the first release of PyPSA-Eur, a model of the European power system at the transmission network level. Recent changes include:

- Documentation on installation, workflows and configuration settings is now available online at
[pypsa-eur.readthedocs.io](pypsa-eur.readthedocs.io) ([\65](https://github.com/PyPSA/pypsa-eur/pull/65)).
- The `conda` environment files were updated and extended ([\81](https://github.com/PyPSA/pypsa-eur/pull/81)).
- The power plant database was updated with extensive filtering options via `pandas.query` functionality
([\84](https://github.com/PyPSA/pypsa-eur/pull/84) and [\#94](https://github.com/PyPSA/pypsa-eur/pull/94)).
- Continuous integration testing with [Travis CI](https://travis-ci.org) is now included for Linux, Mac and Windows ([\#82](https://github.com/PyPSA/pypsa-eur/pull/82)).
- Data dependencies were moved to [zenodo](https://zenodo.org/) and are now versioned ([\#60](https://github.com/PyPSA/pypsa-eur/issues/60)).
- Data dependencies are now retrieved directly from within the snakemake workflow
([\86](https://github.com/PyPSA/pypsa-eur/pull/86)).
- Emission prices can be added to marginal costs of generators through the keyworks `Ep` in the `{opts}` wildcard
([\100](https://github.com/PyPSA/pypsa-eur/pull/100)).
- An option is introduced to add extendable nuclear power plants to the network ([\98](https://github.com/PyPSA/pypsa-eur/pull/98)).
- Focus weights can now be specified for particular countries for the network clustering, which allows to set a proportion of the total number of clusters for particular countries ([\87](https://github.com/PyPSA/pypsa-eur/pull/87)).
- A new rule `add_extra_components` allows to add additional components to the network only after clustering. It is thereby possible to model storage units (e.g. battery and hydrogen) in more detail via a combination of `Store`, `Link` and `Bus` elements ([\97](https://github.com/PyPSA/pypsa-eur/pull/97)).
- Hydrogen pipelines (including cost assumptions) can now be added alongside clustered network connections in the rule `add_extra_components` . Set `electricity: extendable_carriers: Link: [H2 pipeline]` and ensure hydrogen storage is modelled as a `Store`. This is a first simplified stage ([\108](https://github.com/PyPSA/pypsa-eur/pull/108)).
- Logfiles for all rules of the `snakemake` workflow are now written in the folder `log/` ([\102](https://github.com/PyPSA/pypsa-eur/pull/102)).
- The new function `_helpers.mock_snakemake` creates a `snakemake` object which mimics the actual `snakemake` object produced by workflow by parsing the `Snakefile` and setting all paths for inputs, outputs, and logs. This allows running all scripts within a (I)python terminal (or just by calling `python <script-name>`) and thereby facilitates developing and debugging scripts significantly ([\107](https://github.com/PyPSA/pypsa-eur/pull/107)).

What's Changed
* doc: minor fix in API reference by lumbric in https://github.com/PyPSA/linopy/pull/49
* pips: add combined row labels for submatrices beside each other by FabianHofmann in https://github.com/PyPSA/linopy/pull/50
* [pre-commit.ci] pre-commit autoupdate by pre-commit-ci in https://github.com/PyPSA/linopy/pull/52
* fix operands / specify error messages by FabianHofmann in https://github.com/PyPSA/linopy/pull/58
* Replace numpy isscalar check by pandas isscalar check by FabianHofmann in https://github.com/PyPSA/linopy/pull/65
* Allow sum() function over scalar objects by FabianHofmann in https://github.com/PyPSA/linopy/pull/64
* expression / constraints: allow returning None type when creating from rule by FabianHofmann in https://github.com/PyPSA/linopy/pull/66
* [pre-commit.ci] pre-commit autoupdate by pre-commit-ci in https://github.com/PyPSA/linopy/pull/56
* [pre-commit.ci] pre-commit autoupdate by pre-commit-ci in https://github.com/PyPSA/linopy/pull/67
* Define field as default_factory to specify mutable default value by danuriegas in https://github.com/PyPSA/linopy/pull/68
* pips-io: print out rows by FabianHofmann in https://github.com/PyPSA/linopy/pull/69

New Contributors
* danuriegas made their first contribution in https://github.com/PyPSA/linopy/pull/68

**Full Changelog**: https://github.com/PyPSA/linopy/compare/v0.0.14...v0.0.15

**New Features**

* Linopy now uses [highspy](https://pypi.org/project/highspy/) as an interface to the HiGHS solver. This enables a direct and fast communication without needing to write an intermediate LP file.

**New Features**

* The function `LinearExpression.from_tuples` now allows `ScalarVariable` as input.
* For compatibility reasons, the function `groupby_sum` now allows `pandas.Series` as input.

**Bug Fixes**

* Filtering out zeros is now an optional feature in the `solve` function. The previous behavior of filtering just before the LP file writing, lead to unexpected errors for constraints with only zero terms.

**New Features**

* A new module was created to export basic mathematical quantities such as `lb`, `ub`, `c` vectors and the `A` matrix. Use it with the `matrices` accessor in `linopy.Model`.
* For `Constraints`` and `Variables`` a `ipython` autocompletion function for getting items was added.
* Inplace updates for constraints are now more flexible.
* AnonymousConstraint can now built from comparison operations of variables with constants, e.g. `x >= 5`.
* The `Model.add_constraints` function now support input of type `ScalarVariable`, `ScalarLinearExpression` and `ScalarConstraint`.
* Terms with zero coefficient are now filtered out before writing to file to avoid unnecessary overhead.
* The function `sanitize_zeros` was added to `Constraints`. Use this to filter out zero coefficient terms.

**Bug Fixes**

* Solving with `gurobi` and `io_api="direct"` lead to wrong objective assignment if the objective contained non-unique variables. This is fixed in this version.