TODO list before public release

Tests!
Docs!
Cross section calculations.
Field calculations.
Complex frequencies, n's, k's.
Transforming point (meta)generators.
Check whether moble's quaternions and my quaternions give the same results in tmatrices.py
Ewald summations of all types of lattices (dimensionality-wise).
Split lattices.h into separate point generator and lattice vector manipulation parts.
- Maybe move something from the .h to .c file.
Check exact normalisation convention of scuff-tmatrix output.
Check whether the Condon-Shortley phase affects the form of Wigner matrices.
The xflip, yflip and possible i-factor problem.
General 3D point group symmetries.
- Instead the current hard-coded limited set.
- The generation, finding subgroups etc. should be "easy" with quaternions and stuff, as the set is quite limited, see Wikipedia.
- Not sure about the representations, though.
- As a description of a T-matrix / particle metadata.
Nice CLI for all general enough utilities.
Remove legacy code.
Split qpms_c.pyx.
Reduce compiler warnings.
Prefix all identifiers. Maybe think about a different prefix than qpms?
Consistent indentation and style overall.
Rewrite the parallelized translation matrix, mode problem matrix generators in a way that reuses as much code as possible without copypasting

Nice but less important features

Static, thread-safe caches of constant coefficients + API without the current "calculators".