Improve RTSTRUCT end-capping

[lassoan]

1. End-capping creates nice smooth segments but sometimes it creates unexpected results (see for example https://github.com/SlicerRt/SlicerRT/issues/171#issuecomment-758283111). It should be possible to enable/disable it using a conversion parameter (enabled by default).

2. End-capping creates incorrectly oriented normals at the bottom of the structure. See for example (phantom-ent dataset - https://downgit.github.io/#/home?url=https://github.com/SlicerRt/SlicerRtData/tree/master/eclipse-8.1.20-phantom-ent/Original) brain segment. It is visible when exported to models:

[lassoan]

@Sunderlandkyl it would be great if you could have a look at this, too. Multiple groups would like to use SlicerRT DICOM importers because it is so good. However, it is not nice that exported models have to be fixed up (e.g., using vtkPolyDataNormals).

[Sunderlandkyl]

Ok. End cap normal direction should now be fixed, and I've added a conversion parameter to enabled/disable end capping (default enabled).

I plan to add these additional improvements to the end capping:

• Use vtkContourTriangulator to better handle nested contours
• Add additional conversion parameter to switch between the interpolated end cap and the simple straight end cap.

[lassoan]

Thanks a lot!

Both planned tasks would be really nice.

Maybe instead of an additional conversion parameter, we could have multiple values for endcapping (0=none, 1=smooth, 2=straight).