refactoring of spectrum-submodule:

- split of abstract base classes and implementations -> avg, vs_time
- clean-up of old calculate_xxx(..) functions
- legacy function check_spectrum_arguments() moved to spectrum-submodule -> can be removed in the future/merged with checks in post_init of SpectralArguments!
- more alignments in types.py: avoid confusion with naming of protocol classes ("interfaces") -> using leading "I"-naming like in Delphi/.NET
- removed left-overs of numba in P.56

- misc. updates/fixes/improvements