Open and Non-planar curves treated as planar closed curves, with no warning or error

[IsakNaslundBh]

Description:

Open planar curves runs with no errors and give non-sensical results, like:

non-planar curves also runs with no error and warnings, and gives results of a curve projected to a arbitrary plane with the normal och the average plane of the input curve. This should at least be warned for:

Steps to reproduce:

Use the Offset method with a open curve or a non-planar curve

Expected behaviour:

For Open curves to raise an error and return null

For non-planar curves to at least raise a warning.

Test file(s):