Resubmission: Update knot vector synthesis for approximate_curve and approximate_surface

[GarthSnyder]

This is a replacement for PR #121, which ended up having a conflicting branch name. It doesn't look like it's possible to substitute the submitted branch within the GitHub system.

This PR fixes #119 and #120. It includes tests to verify that knot vectors are well-formed, that every knot span includes at least one new control point, and that knot vectors generated for reversed curves are the inverses of those generated in the opposite direction.