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.

[orbingol]

Thank you very much for resubmitting the PR. I've merged some changes to the 5.x branch, so this PR might need an update. Tests are failing due to some changes I merged from 5.x-dev branch. After fixing them, I'll take a look at this PR.

[orbingol]

Also, you might need to install pre-commit (e.g. pip install pre-commit) and run via pre-commit run --all-files to apply the suggestions of the configured code formatter. This is not documented at this time, just wanted to explicitly mention about it.

[GarthSnyder]

Thanks, I will make a detailed review. I'm away for the Memorial Day weekend but will look at this next week.