Proposal: port geometry functions to new repo

[chrisdicaprio]

As a user and developer of nzshm-common I want to eliminate dependencies and make the library pure python. This simplifies installation and reduces dependency creep for libraries and applications as many of our other python projects have nzhsm-common as a dependency.

Currently nzshm-common contains geometry functions which depend on the shapely library. We propose to port these functions to a new repo (possibly nzshm-geometry) to restore nzshm-common to pure python.