Merged PR 42, which implements TH3.
It also introduces the following **breaking changes** in TH2:
* `xedges`, `yedges` are swapped (to their correct order)
* `.numpy()` returns a nested structure, like `numpy.histogramdd`, unlike `numpy.histogram2d`:
python
bin_content, (xedges, yedges) = th2.numpy()
for consistency with
python
bin_content, (xedges, yedges, zedges) = th3.numpy()