Feature/minmax

[gscano]

This is an implementation of a min-max heap as described in the paper 'Min-Max Heaps and Generalized Priority Queue' by Atkinson et al..
Because I extended this data structure to the arity aspect, I based my work on the D-ary heap for the general structure of the class.
All regular tests as well as stability and mutability tests are integrated.

I just want to point your attention towards the modifications I made, for the sake of iterators (ordered and reverse ordered), to mutable_heap.hpp and ordered_adaptor_iterator.hpp.
Because the function get_child_nodes had to return more than one range of indexes (as min and max levels are intertwined within the tree) I had to design an extended_ordered_adaptor_iterator inheriting from ordered_adaptor_iterator and thus created a global ordered_adaptor_iterator_facade to avoid code duplication between those two. The same applies to priority_queue_mutable_wrapper inner structure ordered_iterator.