[disc][porousmediumflow] Use discretization tag instead of enum as template argument.

I've tried squelching the warnings by substituting all comparisons with enums by comparisons with tags, see MR !2891 (merged). Unfortunately, there still were too many warnings. One particular warning concerning tags in porousmediumflow/fluxvariablescachefiller.hh occured very often. Therefore, in this MR I would like to try if it is of any help to fix this warning first. Sorry for the three pending MRs.