Add support for surrogate production over a reduced basis

Currently heron only supports reproducing time-domain waveforms in the time domain, which can be slow.

It's time to develop models which use alternative basis projections to make (much) faster surrogates.

• Add a general reduced basis class
• Add abscissa (linear) interpolation to allow compatibility with bilby interface
• Add tests for the basic class
• Add tests for the sample BBH model
• Document the sample BBH model