Allow the generator to return _id.

[RemiLehe]

Some generators need unique IDs for each points:

• Ax generators. (My understanding is that this is needed e.g. to do parallel, asynchronous bayesian optimization)
• Some libEnsemble generators (including aposmm)

[jlnav]

I don't remember if we discussed this, but are there any assertions on how IDs should be structured? Ought they to be sequential integers, unique hashes, etc. ?