[ENH] probability distributions - boilerplate refactor

[fkiraly]

Towards #251.

This PR refactors the probability distributions, inheriting from BaseDistribution:

• adds a boilerplate layer dealing with input coercion, formatting, indices
• extension points move from public to private under boilerplate, e.g., to _pdf from pdf
  • an extension template is added, detailing the extension contract, fixes [DOC] extension template for probability distributions #23
• distributions are now polymorphic - if all parameters are scalar, and no index is given, behaves as a scalar. Otherwise behaves as a 2D array distribution, as before.

Also refactors all distributions with "non-complex" signature, and also with exception of QPD (which is subject to multiple other PRs)

[fkiraly]

Note to self: refactor is downwards compatible, but default broadcasting cannot deal with the empirical distribution, where parameters are multi-indexed DataFrame.