Minor fixes to the plotting and improve frequency_mask

Closes #338 (closed)

@vivien, this makes the created figures only show the masked data.

I did experiment with make it so that ifo.frequency_domain_strain automatically applied the frequency mask. However, in practise there are a lot of sub routines which are dependent on this. We could add a masked_frequency_array and masked_frequency_domain_strain for convenience to the user, but I think it will be error-prone to change the default behaviour of frequency_array and frequency_domain_strain at this stage.