Spectral margin for aperture spectrum

As discussed, let's change the default spectral margin for the aperture spectrum to be equal to the full spectral width of the source, with a minimum margin of 10 channels. This will make for more useful diagnostic plots with SIP or other tools.

The parameter could be the same as now, output.marginAperSpec, but we set the default value to -1 to get the behaviour described above.

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