ENH: Adding Mixture-Class for superposition of priors

[Hen42rik]

This is a new PR for the same suggested addition of #1077, but addressing the following additional aspects:

1. as requested by @fgittins , the Prior class is renamed to Mixture
2. as requested by @fgittins , it is refactored to inherit most methods from the Interped class
3. the interface is adjusted to respect the new xp_wrap decorators

The original

[Hen42rik]

I would consider this now complete, but happy to consider further remarks @fgittins @mj-will
P.S: Sorry for the delay, holiday

[GregoryAshton]

@Hen42rik this also seems to be doing some rewriting of _split_repr which I think was used to ensure the written repr could be used to reconstruct the prior. I do see extensions to the tests, but I'm trying to figure out if there are any edge cases we need to add. Just leaving this as a comment to discuss

[Hen42rik]

@GregoryAshton I agree that some pitfalls might be hidden here. I have spent some extra time to make sure some of the previously skipped prior tests are now performed, too, but I am not able to vow for this to not break something. I think someone more familiar with these edge cases would probably be more suitable to suggest additional required tests.