Integrate Topology into TileUnit class

Currently the Topology class is only loosely linked to the rest of the package. That may be completely OK.

But... some thought should be given to the integration. Perhaps all TileUnit objects should instantiate a Topology and store it as an attribute. Perhaps they only need to do that if changes which require a Topology are being carried out.

WeaveUnits with spacing < 1.0 can't sustain a Topology, so this also needs to be considered.

Confirmed that WeaveUnits with spacing < 1.0 can't handle a Topology as the code is currently written. Complicated weaves with spacing 1 also struggle.