Effmass

- Fixes incompatibilities with matplotlib 3.3.3 that removed the `warn` keyword from the `matplotlib.use()` method.

- Fixes incompatibilities introduced into previous releases of vasppy (post 0.4.0.12) that changed the internal data structures for k-point and band data, and the API for interacting with these.
- Takes advantage of new vasppy.Procar.from_file() and vasppy.Procar.from_files() methods, in particular to allow reading of multiple PROCAR files as a single dataset.
- Passes additional keyword arguments to vasppy.Procar.from_file() to take advantage of e.g. selecting only zero-weighted k-points.
- Minimum vasppy version set to >=0.5.0.0.

This is the version accepted, after peer-review, to the Journal of Open Source Software.

The key update is that it is able to analyse spin-polarised data. The previous release was able to analyse spin-orbit calculations (LSORBIT=.TRUE.) but not spin-polarised calculations (ISPIN=2).

This release includes bug fixes and improvements as suggested as part of the JOSS review process:

- import correct libraries to run `optical_effmass_kane_dispersion`
- write test for `optical_effmass_kane_dispersion`
- add code of conduct
- add a section to the documentation (readthedocs) that outlines which parts of the package are unique to effmass