Add _Axis.getattr_from and _Axis.getitem_from.

[thetorpedodog]

This aims to eliminate the annoyance (and potential error) of writing

thing = whatever.obs if axis is _Axis.OBS else whatever.var

by letting you say

thing = axis.getattr_from(whatever)

instead.

[bkmartinjr]

OK w/ me as long as we are vigilant about using it!

Afterthought: I wonder if the name could be more self-explanatory? E.g., get_axis_attr or some such?

[ebezzi]

@thetorpedodog merging this PR so that I can use it in #179. Thanks!