Handle the case when the stripper has more than 1 line

[VictorLamoine]

In BezierGrindingSurfacing::generateTrajectory if the vtkStripper (after slicing) has more than 1 line we need to handle that case carefully.

It means that either there is a bug elsewhere or simply that the mesh has a hole.
We need to handle this case both for the grinding/extrication lines.

Possible cases:

• Grinding line has multiple lines, extrication has 1 line
• Grinding line has 1 line, extrication has multiple lines
• Both grinding/extrication have more than 1 line