enable PSD to be specified from an array

This is a pretty simple MR that allows a user to instantiate a PowerSpectralDensity object using arrays rather than a file.

It makes a nice symmetry with Interferometer.set_data.

This would make generating noise realisations easier, for e.g., stochastic signals, I'm not sure of an immediate use for actually running pe.

@gregory.ashton are you happy to merge this.