Add support of log-scale Spectrogram with Daniell method

After adding support of Spectrogram with various methods in !48 (merged), one small bug remained: as most others there, it has to do with GWpy assuming regular frequency axis. In that case it manifests in Ayrray2D class, and here's a quick patch:

diff gwpy/types/array2dOLD.py gwpy/types/array2d.py
352,355c352
<             kwargs.setdefault('xlim', (self.dx.value, self.xspan[1]))
---
>             try:
>                 kwargs.setdefault('xlim', (self.dx.value, self.xspan[1]))
>             except AttributeError:
>                 kwargs.setdefault('xlim', (self.xindex.value[1], self.xspan[1]))

This would prevent it throwing AttributeError with default range, where it just assumes that dx is regular. A proper fix must include wrapping this into Spicypy without touching GWpy file, of course