Added arrayForEachDim method to simplify template metaprogramming.

[odlomax]

This PR adds the following method to src/atlas/array/helpers/ArrayForEach.h:

/// brief   Construct ArrayForEach and call apply
///
/// details Construct an ArrayForEach<ItrDims...> using std::integer_sequence
///         <int, ItrDims...>. Remaining arguments are forwarded to apply
///         method.
template <int... ItrDims, typename... Args>
void arrayForEachDim(std::integer_sequence<int, ItrDims...>, Args&&... args) {
  ArrayForEach<ItrDims...>::apply(std::forward<Args>(args)...);
}

Relevant tests are added to src/tests/array/test_array_foreach.cc.

This feature supports #168 and should be merged beforehand.

[wdeconinck]

Great, no comments :)