Add adaptive adaptation (ha!)

[nsiccha]

I think it's pretty clear that the current user facing interface (in e.g. sample) to e.g. "sample with NUTS with some user-defined number of warm-up and sampling transitions" can be inefficient.

Instead of adding Nutpie/PyMC's warm-up schedule as e.g. mentioned in #311 or implemented in #312, implementing something that stops adapting whenever enough information has been gathered would be more useful - if we get it to work well.