Add slicing support for category axis

[HDembinski]

category axes are currently not sliceable (see reduce algorithm), because slicing didn't seem to make sense for category axes.

However,
scikit-hep/boost-histogram#296
suggests that also category axis should be sliceable, and eventually one should be able to pick individual bin indices as well. Perhaps using the keyword pick(index1, index2, ...) as suggested by @henryiii

[henryiii]

Note on pick: you need both an index and an axes number, since the difference with .at() is that you will leave some axes alone and get a histogram back. Or some way to indicate that an axes should be kept. It would be quite useful for integer (+boolean) axes and category axes, I believe.

[HDembinski]

The patch for making category axis sliceable is almost done
HDembinski#103

[HDembinski]

closed in develop