TODO list before public release

Tests!
Docs!
Cross section calculations.
Field calculations.
Complex frequencies, n's, k's.
Transforming point (meta)generators.
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.
Prefix all identifiers. Maybe think about a different prefix than qpms?
Consistent indentation and style overall.