Simple fix for incorrect "wraparound" catalog properties

Here's a first crack at addressing #120, which noted that when a structure "wraps" around in coordinate space, its computed properties are broken.

This implementation (which is currently overly simple) checks to see if `dendrogram.neighbours` is anything other than the vanilla `Dendrogram.neighbours`. If it is, it assumes that you want to wrap in longitude space. Whenever it finds that you're computing stats on a structure that touches the "minimum" and "maximum" extents of the data, it assumes that it's a wraparound structure and shifts the bottom indices up by the length of the data. Later I'll want to make sure that the resultant computed properties (such as the x_cen, whatever) are within the bounds of the data (so e.g. I'd mod them by `wrap_longitude`)

For control, I've introduced an extra keyword "wrap_longitude" which percolates from `ppv_catalog` down to `ScalarStatistic`, but this wouldn't be the permanent name or form of such a variable.

Any thoughts on whether this implementation is heading in the right direction are welcome.
They are definitely not needed anymore for this change. I'll take them out. (I could see having "neighbours" as a public attribute useful for some imagined use-cases, but that probably doesn't justify including such a change here and risking merge conflicts &c.)