Fix psd

This should resolve https://git.ligo.org/Monash/tupak/issues/103.

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.

These have been removed

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.

These are now properties and the setters update the other.

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.

I think these are a little simpler now. I also got rid of the _set_... functions.

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.

I want to leave this as it is. This interpolation method has been given an underscore to dissuade people from calling it. I'm wary of having interpolation things as properties.