PowerSpectralDensity class cleanup

@colm.talbot @gregory.ashton

Currently there are a few minor issues with the code in PowerSpectralDensity:

frequency_noise_realization is not doing anything -> The intended functionality lives in get_noise_realisation (I think).

interpolated_frequency is not doing anything either and I am not sure what this is supposed to be doing.

amplitude_spectral_density and power_spectral_density are related by a power of 2. We can implement this as a @property to avoid redundant logic in import_amplitude_spectral_density and import_power_spectral_density.

import_amplitude_spectral_density and import_power_spectral_density contain a lot of similar logic that should probably go in a separate static helper method.

interpolate_power_spectral_density could reasonably be implemented as an @property or power_spectral_density_interpolated -> I would check beforehand how often this is called and what performance impact it has.

Maybe it is reasonable to move PowerSpectralDensity into a separate module, but I can see why it would be closely tied together with Interferometer.