Imprecise probabilities

[FriesischScott]

Big discussion for the future

[FriesischScott]

Proposal

Implement all analyses like probability_of_failure with imprecise simulation structs that contain the internal simulation. For example:

pf, cov, samples = probability_of_failure(inputs, model, performance, DoubleLoop(100, MonteCarlo(10000))

Then we implement basic intervals and p-boxes.

@andreaperin and I will try this when I'm back in October.