Julia bugfix in assert and assign params in __modfile__.jl

Created by: sglyon

I found a bug in an @assert call in the first generated dynamic! method in the Julia code (size always returns a tuple, and we were comparing to an int. Dispatch restricts these arguments to all be vectors so length works here)

Also added support for filling in the model.params Vector.

I realize that the Julia support is still preliminary, but I'm happy to pitch in as I have time/need to push things forward.