Adding two tests that reveal an asymmetry in periodic structure finding.
Added two more left/right tests; again, one strangely fails.
ChrisB's fix for periodic_neighbours. Tests pass now!
Merge pull request #125 from tomr-stargazer/periodic_left_right

Asymmetry in periodic structure finding.
Now I put shape_tuple in the metadata and mod xyz by it.

This change alone fixes 2/5 of the `nan` x_cens in Issue #122
I forgot that the last index is one less than .shape -- fixed that!

This one actually makes the problem go away!
Updated test to reflect new "wraparound" modulo-by-shape math.

This is actually the behavior I want, and I'm glad the test case broke when I changed how the math worked.
Added what I expect the length of those two dendrograms to be.
Added a test that indicates I don't compute wraparound centers right.
Revert "I forgot that the last index is one less than .shape -- fixed that!"

This reverts commit a06578be4af37ff2f45ba2600894a4cfc064a366.

I realized that, from a pixel-math basis, this change was 100% wrong.
Tentative fix to work around WCS stubbornness at edge. Possibly will revert.