UnitConversionError during TimeSeries.append() usage

Created by: mikelovskij

When using spectrogram.append() or timeseries.append() i often get an exception saying Valueerror: TimeSeries units do not match: None vs undef " or "Timeseries units do not match: None vs undef". The issue seems to be caused by line 441 of gwpy/series.py where a value without unit is copied in a valued value. The issue seems to be easily solved by changing

            try:
                self.xindex[-other.shape[0]:] = other.xindex.value

with

            try:
                self.xindex[-other.shape[0]:] = other.xindex