Implementing toMatrix() methods for many operators. Also got rid of some free functions
but this needs some more work. Almost all cases in PetscMatrixAssemblyTest are compiling
but many still abort. Also refactored PETScSparseMatrix a little so that only one template
argument is necessary now (only the operator).

Added support for non-square sparse matrices.

Some things that are still open:
- get PetscMatrixAssemblyTest green, enabling all test cases
- remove all former createMatrix() free function calls and definitions
- replace matIdx_t with the new index type before/when merging
- check that no PetscInts remain
- remove all the Petsc ifdefs that are not required anymore
- remove the petsc namespace where they technically only wrap sparse matrix stuff
- discuss what to do with the P1*Operator_new versions