Make frequency/time unit system customizable

[ddkohler]

WrightSim.experiment.pulse.wn_to_omega is the factor that connects time domain quantities to their Fourier Transform representation. At the moment, this attribute is a constant, consistent with fs-wavenumber unit convention, but if we allow this number to be customized, we can let the user decide which unit system is desirable.

Desirable pseudocode: WrightSim.declare_units(time='fs', frequency='eV')