This will indicate to the user that the data will not be modified in the
solver. I think that it's not necessary to differentiate between e.g.
MatrixPointer and MatrixConstPointer, because these are implicitly
mutually assignable.

TNL can be compiled without CXX11 support.

Adding explicit template instantiation.
Moving some _impl.h files into implementation dir.

Adding simple template of project with Makefile to examples.
Adding cmake tests for PETSc for testing of solvers.