Frequency units

[rgcoe]

(this is inspired by questions raised by @rebeccamccabe in #311)

• capytaine uses rad/s, wavespectra uses Hz
• our internally stated logic (not stated in documentation at all) is to use rad/s internally for calculations, but have the user interface be Hz
• the code is (mostly) consistent about using Hz for frequency inputs
  • from_impedance actually uses Hz for the freq. input (contrary to some of the statements above) - perhaps you were confused by the usage of ω in the docstrings? - this is definitely not great and we should probably switch these usages of ω to f
  • from_bem actually does use rad/s, because this is what capytaine produces - note however, that since we are usually recommending/illustrating uses our run_bem function that swaps the phase angle convention it would probably be most consistent to have run_bem also swap to Hz (note that this could be taken care of if we were to complete this issue in
    capytaine: Add frequency (Hz) coordinate to datasets capytaine/capytaine#456)