Rename overlapping_axes -> intersect_axes & make overlapping_axes instead the intersection of names

[cooljoseph1]

Changes as described in #108.

Right now haliax.overlapping_axes returns the intersection of two axes specifications, and raises an error if two axes of the same name have different sizes. I contend that this function should be renamed haliax.intersect_axes for two reasons:

It is the direct counterpart of haliax.union_axes
It returns axes, not the axes' names. In particular, it raises an error if two axes are named the same but have different sizes. The word overlapping suggests that it could be used to get the names in common between two axes specs--such as if you want to check if you need to rename any of them. Throwing an error is strange behavior here.
Hence, I suggest that haliax.overlapping_axes be renamed to haliax.intersect_axes, and the function haliax.overlapping_axes be changed to return the intersection of the axes' names.

The main reason to not make this change is that it would break the API, and that might be reason enough to leave it as is.

[dlwh]

thanks!