[parameters] allow the same parameter name within different groups

So far, it was not allowed 
- to have the same parameter name within more than one group,

- to deduct the same parameter name from more than one property.

I assume historical reasons for this. In the early years, there were no
groups at all and these measures should facilitate an easy migration
from without groups to groups. Since this migration has been done for
quite a while now, there are no reasons for keeping these restrictions.
Even more so, since they are rather contradictory to the group concept.
Moreover, assigning a parameter to the wrong group will result in being
listed as "unused parameter" at a very prominent position.

Reviewed by gruenich.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14131 2fb0f335-1f38-0410-981e-8018bf24f1b0