Check that arguments passed via kwargs are valid

Checks added in the stan function and the methods sampling
and optimizing.

Closes #110
Removes Stan code before adding submodule
Add Stan source via git submodule
Update interface for new version of Stan

Support checking for user interrupt during optimizing.
Add var_context_factory.

Closes #93.
Remove unused code from stanfit4model
Merge branch 'feature/stan-develop' into develop