Conflicts:
	src/TNL/Problems/HeatEquationProblem_impl.h
	src/TNL/Solvers/PDE/ExplicitUpdater.h

And of course our implementation is faster ;-)

It cannot work because indices are not initialized to -1 anymore. The
checks in the MeshBuilder class should be enough.

This way GCC does not complain that they are not used in some
compilation units.

It should not be called "base" because inheritance does not work for the
constexpr methods.

It is checked in the ConfigValidator.