Polymorphism in the physics options.

[grey-nearing]

[bartnijssen]

@greyNearing : Grey I am going through the issues just to see which ones we need to complete before we release SUMMA 2. This would not be one of them, but if this is still desired, please elaborate on what you want to see here and if you have any suggestions on how this could be implemented. I assume this is different from #160 and that you actually want a physics function to be able to accept different inputs (of different types).