Illustrative attempt at 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.
This sounds good -- however I don't think the "indices" are in coordinate space yet but rather are in pixels, and so one would need to both know (a) how many pixels across is this data? and (b) are the data meant to wrap around 360-degrees-wise at the edges?