The exception is not used anywhere and does not make much sense.
Functions/methods which are not implemented yet can just throw
std::runtime_error wich a more specific message.

(Jak má člověk propalovat čas v Ostravě, když mu to školitel rozbil....
git log -L60,60:src/TNL/Meshes/GridDetails/Traverser_Grid3D_impl.h)

Even the methods for specializations without storage have to return
something, which was not the case...

This is closer to value_type which is used in STL containers.

Fixed handling of the boundary conditions for explicit solver - introduced a method Problem::applyBoundaryConditions.