Update centroid logic to cacluate once and update code to match stylegui...

Updated test as per comments on initial PR (https://github.com/geokit/geokit/pull/147) by including localized test variables within the test method.

Cached centroid calculation as right now polygons are static and can not have new points added after creation.  I did this on demand as opposed to object initialization, input welcome on maintainer preferences.

Did not resolve hound line length comments but will be happy to do so if requested.  Updated code to reflect style guide comments aside from that.