[FEATURE] Slicing subsets of categorical axes

[pfackeldey]

It would be great to do:

ax
# >> StrCategory(['DY', 'QCD', 'Higgs'], name='process')

# the following currently fails
ax[["DY", "QCD"]]
# >> StrCategory(['DY', 'QCD'], name='process')  # <-- this is what I'm looking for

similarly to how it works on hist.Hist already:

hist = hist.Hist(ax)

hist[["DY", "QCD"]]
# >> Hist(StrCategory(['DY', 'QCD'], name='process'), storage=Double())