Refactor init logic in pm.sample

[jessegrabowski]

Description

We currently have a long list of if/else conditions. but it's needlessly restrictive. Basically the user is allowed to select an initialization mode (jitter, MAP, or advi) and an adaptation mode (adapt_diag, adapt_diag_grad, or adapt_full). Currently some of these combinations are arbitrarily disallowed, for example advi+adapt_diag_grad seems nice.

I propose splitting the incoming string on + and then dispatching to appropriate functions. This would make it easier to implement new initialization or adaptation methods. I can imagine the nutpie fisher loss adaptation, or normalizing flow adaption (or initialization!), being added as choices, for examples.