Improve prior functionality

[raphaelshirley]

As part of the AGN development we want to include addtional prior functionality. As part of this we would like to move the existing prior functionality (abs mag and n of z) into a distinct prior class.

We would also like the prior to be overwritten on an object by object basis via Python. Experiments show this can slow down openmp so we need a system for computing priors outside the openmp loops and or a more general n of z function which can be overwritten via Python.

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