Implement over-allocation for histograms with growing axes

[HDembinski]

Histograms with a growing axes should allocate more space than needed, similar to vector's push_back. This requires adding an extra field on an axis to query the spare capacity. A storage is then only resized if the spare capacity is exceeded.

[HDembinski]

We then need a reserve() on the axis and shrink_to_fit() to the histogram.