[linear] Rename amgparallelhelpers to parallelhelpers, deprecate and cleanup

* createIndexSetAndProjectForAMG -> createParallelIndexSet
* getExtendedMatrix -> extendMatrix

dumux/linear/CMakeLists.txt

@@ -5,6 +5,7 @@ amgtraits.hh
 linearsolvertraits.hh
 linearsolveracceptsmultitypematrix.hh
 matrixconverter.hh
+parallelhelpers.hh
 pdesolver.hh
 scotchbackend.hh
 seqsolverbackend.hh