trap/warn if assign HMC to invalid nodes

[paciorek]

With the changes to AD handling, we are now in the position that a user could assign HMC to invalid nodes. For example, dcat and other discrete nodes or to nodes that have truncated or constraint dependencies.

At the moment we (as of AD-stoch-ind branching being merged into nimble`) simply have a run-time error when we run the HMC.

I think that in assigning NUTS samplers, we should check for invalid cases. I think these are:

• discrete nodes
• nodes assigned truncated distributions
• nodes with dependencies of truncated distributions, dinterval, dconstraint
• user-defined distribution without derivative support (I haven't thought through how difficult this would be)

@danielturek @perrydv Thoughts?

[danielturek]

@paciorek Yes, I think that makes sense about the cases to catch. In the HMC samplers we already were checking for discrete nodes, but not the other cases.

And @perrydv is there a way to determine if a user-defined distribution has AD support?

[danielturek]

Started working on this in PR #42.

[danielturek]

Fixed in PR #42 and #57