Split / merge segments

For use in an interactive tool, there needs to be an ability to split a segment at some point, as well as the ability to take two adjoining segments and join them, adjusting control points so as to approximate the original curve.

The issue names no files, tests, or entry points. First inspect the repository structure and existing spline and segment APIs to find where curve control points are represented. Done means supporting both splitting a segment at a chosen point and joining adjoining segments while approximating the original curve.

Written by the indexing model from the issue text.