Check and improve STL compliance

[HDembinski]

add appropriate tests and exampes

from #103: First, std::distance() does not work on a histogram with array_storage. This means that other STL algorithms, like std::any_of(), fails to compile. I solved this problem by copying the distance_to() function from the axis iterator to the histogram iterator.

Second, std::max_element (and brethren) cannot be used on a histogram. Consider the following example:

auto element = std::max_element(h.begin(), h.end());

Compilation fails for adaptive_storage because weight_counter has no less-than operator. Furthermore, compilation fails for array_storage because iterator_over::operator= fails. It attempts to re-assign a reference, but accidentally triggers a copy-constructor instead. Letting the iterator store a pointer instead of a reference solves the problem.

Apropos, iterator_over::operator= does not return *this. Consider adding the -Werror=return-type compiler flag and a unit-test to that calls this operator.

Third, std::inner_product fails to compile for adaptive_storage because weight_counter does not have a binary operator*. The inner product of two histograms is useful for calculating the cosine similarity, which can be used to compare two distributions. std::inner_product works for array_storage though.

[HDembinski]

All this works now in develop.