Feature/petsc cmake

added cmake modules FindPETSc

cmake/modules/AmdisMacros.cmake

@@ -31,7 +31,7 @@ elseif (BACKEND STREQUAL "ISTL")
     message(FATAL_ERROR "Need dune-istl, MTL, PETSc, or Eigen3 as linear algebra backend. Change flag BACKEND!")
   endif ()
 else ()
-  message(FATAL_ERROR "BACKEND must be one of MTL, EIGEN, ISTL")
+  message(FATAL_ERROR "BACKEND must be one of MTL, EIGEN, PETSC, ISTL")
 endif ()