parameters: make the GET_*_PARAM macros work in non-template contexts

for this we take advantage of the fact that template functions never
need to be disambiguated using the 'template' keyword.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6495 2fb0f335-1f38-0410-981e-8018bf24f1b0