Tangos

*Bug fixes*

* Fix multiple bugs in `earliest()`, `earlier(..)`, `latest()`, `later(..)` live calculations and related functionality
* Issue 77, unreliability when cross-linking different simulations (mrey)
* Compatibility with Python 3.7 and 3.8
* Compute correct `contamination_fraction` when writing in server mode (mrey)
* Fix bug in `__repr__` for Links when `weight` is None
* Fix bug in property writer, where pre-requisites could be erroneously flagged as not present simply because they were links rather than properties

*Features*

* The tolerance for considering two timesteps 'the same time' is now configurable within `config.py`
* Add `==`, `<=` and `>=` comparison operators to live calculation
* Added a tool to import AHF merger trees (tobibu)
* Add ability to import hostHalo/childHalo from AHF
* Extent `abs()` live-calculation function to take absolute value of scalars as well as vector quantities

*Bug fixes*

- Fix a bug where calculations on multiple objects (such as with `calculate_all`, `calculate_for_progenitors` etc) would raise exceptions when no results are available, instead of the documented behaviour of returning an empty result.
- Fix bug in `tangos write` where multiple copies of a property would be written even if `--force` was not specified.

*Enhancements*

- Improvements to live calculation documentation

*Bug fixes*

* Fixed a bug that could lead to inaccurate results when using link searches such as `earlier(n)` or `later(n)` across multiple halos (e.g. using `calculate_all`)

*Enhancements*

* Add a `find_progenitor` live calculation facility, which searches for progenitors giving a maximum or minimum value of a property
* Improved live calculation documentation

*Enhancements*
- Add an option to download merger trees as SVG from the web interface (martin-rey)
- Improve efficiency of `calculate_all` queries and further massive performance improvements to `MultiSourceMultiHop` tree searches by creating a temporary index on the temporary halo link table
- Improve efficiency of `tangos write` MPI sessions, by implementing a shared lock for reading the database (so that exclusive rights are not required except for writing)
- Add support for Eagle-like runs, i.e. GadgetHDF format with halo catalogue as two arrays (groups/subgroups). Add a tutorial using Eagle data.
- A better colour scheme for the merger tree
- Enhance the way that links can be distinguished in the live calculation syntax, and document in `live.md` (mtremmel)
- Improvements to the handling of Changa black holes (mtremmel)

*Bug fixes*
- Fix bug where viewing an array in the web view could lead to an infinite recursion crash

* Bug fix for compatibility with databases created with very old versions of tangos
* Major performance fix for MultiSourceMultiHopStrategy that speeds up tracing multiple halos across multiple timesteps by factors of up to 50

Fixes a bug in StarFormHistogram introduced in Version 1.0.4