Permit sharing y-labels on the right spine, x-labels on the top spine

[lukelbd]

This PR will expand the axis-sharing feature so that it works with tick labels on the left, right, bottom or top spines, not just the left/bottom spines. Figure(..., sharex=sharex) will share groups of x-axes whose tick labels are on the bottom or top with the bottommost and topmost subplots (respectively), and Figure(..., sharey=sharey) will share labels from groups of y-axes whose tick labels are on the left or right with the leftmost and rightmost subplots (respectively). This will address #205.

This will also allow me to tackle #79 (and #178). It is much more common to need longitude/latitude labels on the right and top spines for geographic projections than for Cartesian projections (e.g. this Lambert conformal example requires labels on the top spine for the 60 degree and 80 degree latitude lines, because they don't intersect any other spine). This is a major reason reason I had not tried to implement "axis sharing" for geographic plots yet.

Just wanted to open a PR because it's one of the more important changes to proplot needed before v1. May not have a chance to tackle it for several months.