Restructure and add hyperfine

[johannes-moegerle]

This is a major restructure of the code base.

We split the RydbergState in:

• a RadialState (mainly has all the functionality from as before)
• and an AngularKet/AngularState which now also supports hyperfine and different coupling schemes, see below

AngularKet / AngularState

• AngularKets consist of the quantum numbers i_c, s_c, l_c, s_r, l_r, f_tot and 3 additional qns according to the coupling scheme (e.g. l_tot, s_tot, j_tot for LS coupling)
• implemented functionality for LS, JJ, and FJ coupling scheme, this includes
  • overlaps, also between different coupling schemes
  • transform a state into a different coupling scheme basis
  • calculate the angular matrix elements of typical operators

Related PRs

pairinteraction/MQDT.jl#16
pairinteraction/database-mqdt#7

TODO in future PRs

• rename elements/ to species/; BaseElement -> SpeciesObject; RydbergState allow argument species as str or object; state.element -> state.species, state.species -> state.species.name; in general class ...Base to class ...Object? (see Rename elements to species #40)
• decouple use_nist_data option from the element/species; make a utils function calc_nu(species, n, l, ..., use_nist_data) (see Rename elements to species #40)
• clean up / simplify radial grid.py, wavefunction.py and radial_state.py (see clean up RadialState, Wavefunction, Grid rydstate#3)
• adapt to pairinteraction naming conventions (e.g. operators in small caps, names of calc/get methods ...))