Explicit instantiation of template CUDA reduction.

TNL can be compiled without CXX11 support.

Adding tnl-view tool.

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

Adding directory legacy for old code.

Implementing PDE solvers and time steppers.

Changing explicit solvers - Merson and Euler.

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