Introduce a range_spectrogram method as a figure-of-merit

Created by: alurban

This PR introduces two new functions in gwpy.astro:

• range_timeseries
• range_spectrogram

Unit tests and documentation examples for these new functions are also included.

The range_spectrogram utility works by first calculating a standard PSD spectrogram (or accepting one as an input), then stepping through its time bins, passing each PSD measurement to either the astro.burst_range_spectrum or astro.inspiral_range_psd function depending on the use case. The range_timeseries utility follows a similar procedure, but returns a TimeSeries record of the output of astro.burst_range or astro.inspiral_range.

Notes:

• This PR is mainly to address requests from Advanced LIGO commissioners, who want to analyze spectral contributions to the binary neutron star inspiral range on the LIGO summary pages
• Both methods default to a 1.4-1.4 binary neutron star (BNS) inspiral range, but support either inspiral (of arbitrary masses) or burst (of arbitrary energy) range calculations

cc @duncanmmacleod