consistently index axis from 0 to 2
Merge pull request #403 from ndawe/axis

[WIP] consistently index axis from 0 to 2
fix broken underflow() and overflow() methods
Merge pull request #404 from ndawe/axis

[MRG] fix broken underflow() and overflow() methods
hist: add set_sum_w2 and get_sum_w2 methods
hist: implement merge_bins method to merge arbitrary sets of bin windows