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.

- 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.

Implemented Array:containsOnlyValue.
Restored implementation of ArrayOperations::memoryCompare on host.

Implemented a method Array::checkValue which returns true only if there is an element with given value in the array.

And of course our implementation is faster ;-)

- separate data types for input arrays of binary operations
  (necessary for the equality comparison of two arrays of different
  type)
- added separate template parameter for ResultType, which defaults to
  the RealType
  (the goal is to allow sum of bool vectors into int instead of bool)

- Use math function from TNL/Math.h instead of the standard library
- Many cosmetic changes for readability