Type checking for propagate kwargs.

[GuusAvis]

Added type testing for some kwargs in propagate, and adjusted a test that I think was already failing because of a wrong type but now it no longer fails quietly.

This is basically the same as #126, but applied to the changes of #132 (where an extra kwarg was introduced) and with a fix of the test.

Unfortunately is seems that the changes are triggering the failing of some other tests. I think the problem is that now, in case keep_deltas isa Val{true} but keep_triples isa Val{false}, propagate sometimes returns a Dual type instead of a StochasticTriple type. The test expects the latter though. I'm not entirely sure what the intended behavior is though so I won't make any further changes without further discussion.