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