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.
Formatted whitespace for better visual inspection. No content change.
Added a number of (failing, somehow) extra periodic tests.

These really don't need to be separate tests, we could iterate through some kind of structure instead. Next steps though.
Added several new catalog tests. One currently fails (test_catalog2).