First cut at creating a shared Noms types package, by creating
'geotypes.noms' to define Geoposition and Georectangle. These are
shared by quad_tree and sfcrime_importer initially, but will also
by used by sfcrime_search and the stuff that works with photos.
quad_tree used to add methods to the generated Geoposition and
Georectangle types. This doesn't work anymore, since these types
are now generated as part of a separate Go package. Fortunately,
these methods are easily re-cast as pure functions.
Towards #409
4169f2af3a