Expr output JumpProblem

[ChrisRackauckas]

#466 ignores doing JumpProblem since that might interfere with some of @isaacsas changes, so we can add it afterwards.

[isaacsas]

If you want to get that in now feel free. I can merge / rebase as needed... I probably won't get back to it till this weekend due to other stuff.

[isaacsas]

Also, the changes will primarily be on the mass action jumps, which I'd assume this shouldn't really affect much.