rename `Prior` to `Distribution` or something similar

[williambdean]

Something like Distribution may make a bit more sense with these also having a create_likelihood_variable.

Not pressing as many are used as priors and the Likelihood has the sigma prior roped in with it.

What are people's thoughts on this?

I believe @tomicapretto brought this up in the thread. I will link it when I can.

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This issue was transferred from pymc-labs/pymc-marketing#1345