Add GEOS functions supported by Shapely that GeoDjango lacks

[Paul Winkler]

We don't have interpolate() or project() methods. Shapely does:
​http://toblerity.github.com/shapely/manual.html#linear-referencing-methods

It can useful to specify position along linear features such as LineStrings and
MultiLineStrings with a 1-dimensional referencing system. Shapely supports linear referencing based on length or distance,
evaluating the distance along a geometric object to the projection of a given point, or the point
at a given distance along the object.

I don't know what else, if anything, Shapely has that we don't have, but I stumbled on not having interpolate today.

[Claude Paroz]

Duplicate of #11948