Improve shared axis label algorithm

[pulkin]

The issue is, for complex layouts such as the one below, proplot screws up shared axes labels positions.

   1111111
L  1111111
A
B  1111111
E  1111111
L
1  222 222
   222 222
   222 222

In the above case, LABEL1 occupies the entire column even though the second group of plots might have its own shared and different label axis. The patch addresses this case and checks whether label text is the same across subject axes.

Further work is needed in this direction to develop a definitive policy regarding axes label grouping.