Add Constraint prior

This MR adds a new Constraint prior class.

This allows users to specify bounds in some parameter which isn't being sampled, e.g., when using the ROQ one wants limits on chirp mass and mass ratio, #320 (closed).

This will also allow us to remove the pretty ugly way we enforce mass ratios being less than one.

This involved digging into a decent amount of low-level code but can be entirely backwards compatible. Although I'd like to make a non-backwards compatible change to the source module, specifically removing the hardcoded m1 >= m2.