Pyelli

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

- Adds reading functionality for [NeXus files](https://fairmat-experimental.github.io/nexus-fairmat-proposal/50433d9039b3f33299bab338998acb5335cd8951/classes/contributed_definitions/NXellipsometry.html#nxellipsometry).

- The fitting module is not imported at top-level anymore. It has now to be imported by `elli.fitting`.

- Dispersions are now addressed by their name only (instead of Dispersion...)
- Dispersions are initialized with two distinguished set of parameters for parameters which are set once and parameters which may be set multiple times (for oscillators etc). They can be added by invoking the `add` command on the respective class.
- There is a new factory class `DispersionFactory` to get a dispersion from it's string name, i.e. `DispersionFactory.get_dispersion(...)`