gwpy.timeseries restructuring
This PR restructures the gwpy.timeseries module to provide a (fairly) abstract base class for the TimeSeries and friends.
This should mean that the StateVector and StateTimeSeries classes are no longer bloated with extra Fourier-style methods that they can't use.
Additionally, this PR fixes a minor bug in the way args and kwargs (including axis) are passed to Spectrogram ufunc methods (discovered in testing the above changes).