Fixing category indexing

[henryiii]

Fix issues with indexing and summing category axes. You can now sum over a category axes again (regression in 0.6.0), and you can select a single item from a category axes just like you can from other axes types.

Fixes #297, improves #296 (single item selection now works on a category axis).

This may be improved upstream in Boost.Histogram later, in which case this can be simplified eventually.

[henryiii]

@HDembinski Let me know if this seems like a reasonable workaround for 0.6.2; I assume Boost.Histogram will later add utilities that will make this easier. A single item no longer becomes a slice of length 1, and the trick shown on Gitter is used to produce the output. This also fixes sums over category axes, which were trying to slice in 0.6 and so were failing.

TODO: This internally tests sums over category axes in the tests now, but that should be added as an explicit test in the future to be safe when the algorithm changes.