bug in is_periodically_contiguous( self ):

[connorourke]

Bug in is_periodically_contiguous:

Returns True for a 2 site (and other sizes) cluster in the centre of the lattice that doesn't connect across the boundary, as the sites are both at the edges and neigbouring one another.

[bjmorgan]

Good spot. Now that I can see it, it's obvious that the original method will give the wrong result for a 2-site cluster.

I've added a (failing) test for this case. bd5ffb3