Group arguments in assembly rountines

I think fv element geometry, elem volvars and flux vars cache belong together in an element-wise assembly view point. This could be expressed, e.g. by grouping these objects together. This might reduce the number of arguments for some functions. It's not completely trivial since there are several sensible combination possible.

I added a suggestion for an element view in !2125 (closed). Suggestions for improvement? What that speaks for/against this?