Madminer

Bug fixes:
- Fixed particles `phi` property access after [scikit-hep/vector](https://github.com/scikit-hep/vector) migration (not callable anymore).
- Fixed missing _"modified_score"_ mode within `Ensemble.calculate_fisher_information()` accepted ones.

Internal changes:
- Improved `math` functions mappings within the utilities (https://github.com/diana-hep/madminer/commit/6a7ead35ece83759002e1402ebbe48da882443c6).
- Replaced most of the _hardcoded_ particle IDs by the [scikit-hep/particle](https://github.com/scikit-hep/particle) package.
- Removed legacy `check_required_data` utilities function (https://github.com/diana-hep/madminer/commit/13bf559a3d7ad0614bc96df66bc2804ed6d53fb7).
- Removed legacy `UploadCommand` setup.py class (now using GHA workflows) (https://github.com/diana-hep/madminer/commit/243e1e80d0e72e5eddf626609be52f2ac6c46c59).

Documentation:
- Defined new `CITATION.cff` [file](https://github.com/diana-hep/madminer/blob/master/CITATION.cff) to ease package citation.

Bug fixes:
- Fixed MadminerParticle initialization by using classmethod helpers. Thanks lyazj.
- Replaced `MadminerParticle` inherited class (`VectorObject4D` -> `MomentumObject4D`). Thanks lyazj.
- Bumped `vector` package to version `0.8.4`, first version to properly allow subclassing.

Internal changes:
- Replaced Python2 compatible super-class calls by Python3 style.

Documentation:
- Fixed README relative links (unavailable on [Madminer's PyPi information page](https://pypi.org/project/madminer/)).

Bug fixes:
- Fixed a Python2 -> Python3 migration issue when dealing with dictionary values (thanks sbrass).

Internal changes:
- Replaced legacy `scikit-hep>=0.5.0, <0.6.0` requirement (underused), by the [`vector` package](https://pypi.org/project/vector/).
- Replaced legacy [rootproject/root-ubuntu](https://hub.docker.com/r/rootproject/root-ubuntu) Docker base image by [rootproject/root](https://hub.docker.com/r/rootproject/root/) equivalent.

Documentation:
- Fixed [readthedocs](https://readthedocs.org/) automatic documentation building (broken since July 1st 2020).
- Define explicit `.readthedocs.yml` file with the desired configuration.
- Define GitHub `docs.yml` workflow, to validate documentation building upon `master` branch pushes / PRs.

Internal changes:
- Dropped legacy Python 2 support. MadMiner now only supports Python 3.6+.
- Replace string templating to make use of Python 3.6 [f-strings](https://www.python.org/dev/peps/pep-0498/).
- Fixed typos and doc-strings inconsistencies throughout the codebase.

For more information: https://github.com/diana-hep/madminer/pull/438

New features:
- Prototype implementation of joint score computations with finite differences (with `MadMiner.finite_differences()`)

Bug fixes:
- Fixed event loading breaking with h5py version 3.0

Internal changes:
- Improved platform independence by using the `gzip` module rather than calling the `gunzip` program

Documentation:
- New tutorial notebook for the finite difference setup

New features:
- New keyword `order` in `MadMiner.run()`, which can be set to 'nlo' to set the systematics card in a way compatible with NLO event generation (which remains not really supported for now)
- Preliminary implementation of the quadratic model proposed by https://arxiv.org/abs/2007.10356

Bug fixes:
- Fixed a critical bug in the morphing-aware likelihood ratio estimator