Fem: Improve constraint section print

[marioalexis84]

• Add Variable property to select CalculiX section print key. (Drag Stress not yet supported)
• Add symbol to the view provider.
• Update task panel.

@FEA-eng

[FEA-eng]

Nice, recently I've noticed that this constraint has an empty SectionPrint parameter panel and I was going to suggest that it should be removed but you made a good use of it as more options may now be supported since heat flux will be available in thermal analyses.

I would just like to suggest renaming Section Load (suggesting external loads) to something like Section Forces (even if moments are included as well) or Section Forces and Moments. When it comes to Drag Stresses, we may even omit them from the selection and leave only the other 2 options.

[marioalexis84]

Enum changed to Section Force.
I prefer not to omit drag stress even if it is not used. If we add support for fluids in the future, a workaround is needed to update the old section print objects with the new value. It could be added to the documentation that the option is not yet supported.

[FEA-eng]

I prefer not to omit drag stress even if it is not used. If we add support for fluids in the future, a workaround is needed to update the old section print objects with the new value. It could be added to the documentation that the option is not yet supported.

Ok, I guess that we can keep one not yet supported property value.