Calculate power spectra using a separate operator power_spectrum

[cehalliwell]

Follows on from #1765

The power spectra is currently calculated and plotted in the main plot.py operator. However, this method does not allow aggregation of the power spectra (e.g. mean). The power spectra are calculated in a separate operator rather than in plot.py and the resulting power spectra plotted using the line series method in plot.py. After this, aggregation will be possible using the collapse operator.

Move the existing functions and code for calculating the power spectra into a separate operator, remove the existing power spectra code from plot.py and adjust the line series plotting code to allow for line plotting other than timeseries.

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