This new version of binary_find needs a equal operator between each
parameters (ForwardIterator and Value).

This patch introduces a irt::fixed_real_array<size_t> template structure with
the same behaviour than irt::message. This new template structure is used for
message, dated_message and observation_message. The size of the irt::message is
only three reals while irt::dated_message and irt::observation_message use four
reals.