Feature/richards without switchable privars

What this MR does / why does DuMux need it:

Allows to be able to use Richards without Switchable privars (was hard-coded before). Switchable privars are only needed if a Privarswitch is used. This should really be optional. In order to ease a transition, this MR allows Richards to be used without Switchable privars but does not change the default behaviour. Also see #1165 (closed).