Investigate the issues with the remove knot implementation

[orbingol]

It looks like there is a problem with the remove knot implementation. I'd like to investigate it a bit more and fix it. However, I don't want to block the release with some other fixes.

Let me add the previously opened issues here as a list.

• helper.knot_removal returns incorrect result (+ fix) #176
• remove_knot implementation bug #135
• Degree elevation modifies NURBS curve shape #125
• Update degree elevation and reduction functions #44