``NurbsCurve`` checks

[sonomirco]

Checks the following method for closed / periodic curves.

• Offset
• Divide
• PerpendicularFrames
• SplitAt
• SubCurve

[d3ssy]

Specify what should happen when checks pass and when checks fail for each method (e.g. SplitAt with a closed curve should throw, return null, or return the original input?)

[sonomirco]

@d3ssy
So all these functions don't check if the NURBS curve is closed (periodic).
So if it is closed has to be clamped first to make the curve usable.

1. Offset should be able to make the operation even if it is closed.
2. Divide should clamp the curve first.
3. PerpendicularFrames should clamp the curve first.
4. SplitAt should return a closed curve.
5. SubCurve should clamp the curve first.