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

The original was simpler/more readable and we require GCC 5.0 now.

This reverts commit d920a278.

The std::isnan method can be safely used in host code (if the compiler
is not moron). There is no device code which uses isnan so the
TNL::isNan function is useless.

This reverts commit 0eff2f63.

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

It has not been implemented in the past decade so it can be safely
assumed that it is useless.

- Replaced Constants.h with std::numeric_limits, see also
  https://jlk.fjfi.cvut.cz/gitlab/mmg/tnl-dev/issues/3
- Made the initialValue method of reduction opeerations static and
  constexpr.

Added assertions to MpiCommunicator.h checking that MPI functions are not called with the null group