coreferents() function to find tiers/items aligning to the same thing

[goodmami]

In some cases, multiple kinds of annotations can align to the same thing, but not transitively. That is, instead of A->B->C, we may have A->C and B->C. In these cases, it would be useful to be able to find what items on A and B refer to the same things in C.

Maybe something like:

>>> query.coreferents(tier_a, tier_b, 'alignment')
[(<Item (id: a1) at ...>, <Item (id: b1) at ...>, [<Item (id: c1) at ...>]), ...]
>>> query.coreferrers(tier_c, 'alignment')
[(<Item (id: c1) at ...>, [<Item (id: a1) at ...>, ...], [<Item (id: b1) at ...>, ...]), ...]

[goodmami]

This won't make it in time for the 1.1 release. Removing the milestone.