According to benchmarks, there is practically no difference in
performance. Only explicit unrolling is helpful, but that has been
implemented for the general algorithm in Reduction::reduce as well.

Now that error handling is done via exceptions, the return value is free
for more sensible things.

The method was either returning true or throwing an exception, so it can
simply return void or throw an exception.

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

This reverts commit d920a278.

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.

Also added missing 'const' to the getMemoryElement method for host.

[skip ci]

They are virtual, currently not needed and only complicate things.