Make FrequencySeries.ifft() consistent with TimeSeries.fft()
Created by: alurban
I've taken a stab at altering the units and normalization of FrequencySeries.ifft()
so that it reproduces the original timeseries after TimeSeries.fft().ifft()
. I've also added a unit test to make sure this is the case.
This fixes https://github.com/gwpy/gwpy/issues/729.