Pyelli

New
- Include the Refractiveindex.info database and import and search scripts to access it
- Kramers-Kronig-Relationship conversions for dielectric functions
- Add Cody-Lorentz dispersion model

Relevant pull requests
* Kramers-Kronig relations by domna in https://github.com/PyEllips/pyElli/pull/89
* Basic refractiveindex.info importer by MarJMue in https://github.com/PyEllips/pyElli/pull/87


**Full Changelog**: https://github.com/PyEllips/pyElli/compare/v0.14.1...v0.15.0

Bugfixes

* Installs the correct extra requirements in the docker container

Breaking changes

* Moved importers into the importers submodule
* Split the spectraray class into a class to load a dispersion table and an importer

Bug fixes

* Introduced proper dependency management
* Changed the dependency of extra install requirements for fitting and testing

Breaking Changes:
- Moved math submodule into utils, to avoid name conflict with Python's math module

Bug fixes:
- Fix error in eigenvalue sorting (PropergatorEig and non-isotropic backmaterials)
- Pin version of ipywidgets to keep plotly working

Breaking Changes:
- Renamed the conversion functions and added more
- result.R and .T now return the reflectance/transmittance instead of the respective matrix, which can be accessed with .R_matrix/.T_matrix
- Renamed PropagatorExpmScipy to PropagatorExpm
- Removed Torch and Tensorflow Solvers

New:
- Added a lot of documentation
- Added a Bruggeman EMA Material
- Support for transmissive Ellipsometry

Bug fixes:
- Fix nan values in MaxwellGarnettEMA

What's Changed
* Adds [NeXus](https://fairmat-experimental.github.io/nexus-fairmat-proposal/50433d9039b3f33299bab338998acb5335cd8951/classes/contributed_definitions/NXellipsometry.html#nxellipsometry) reader capabilities.
* Small bugfixes