Generalized initial uncertainty

[aml5600]

Currently the interface supports uncertainty via elements of the initial state/parameters being "sampleable" (distributions). This is limiting in the sense that it assumes that the dimensions of the initial joint pdf are all independent.

I think the interface would benefit from a more general approach, possibly something similar to the prob_func arg from EnsembleProblem? However, instead of returning a new problem it returns a tuple holding an initial joint pdf draw (u0) and it's associated value f0(u0).

[aml5600]

FYI @agerlach