single suspenction seismic noise function
This refactors the seismic noise calculation to be for a single suspension. The turning of that into a seismic noise spectrum for a LIGO-like IFO is moved to the Seismic(nb.Noise) class definition in the ifo module, effectively separating fundamental seismic noise calculation from assumptions of experimental configuration.
This is a straw-man for a path to separate assumptions of experimental configuration from the noise calculators. If we like this way of doing things I'll continue to do this kind of thing for the rest of the noise calculators and budget noise sources.