Feature Request: Allow export of helical angle field

[zaracay]

Similar to allowing the sizing field export, it would be nice to export the helical angle field as it is attached to the mesh. This would allow the user to port it into SCIRun for further analysis, which does not currently have a helical angle quality measure implemented.

I wouldn't make it prompt the user for it upon exiting the program, but just have an option to export it in the File dropdown.