BUGFIX: relative binning resolution and reference parameter caching

Various users have identified two bugs in the relative binning implementation.

• when specifying the fiducial parameters in terms of component masses and sampling in chirp mass, the mass posteriors don't make sense. To fix this, I manually clear the stored parameters at the end of the init.
• for small epsilon the bin width can be less than the data frequency spacing. I manually imposed that a bin must be at least as wide as the frequency spacing.