Additional results deformation parameter for postprocessing

Currently results plot can be shown deformed according to several parameters: total displacements, forces, surface normals, etc. It would be nice to add stress to this list of parameters. This allows for a 2D model to show stress as a 3D surface
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instead of the standard 2D plot
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This feature is more useful for 2D and plate models for which the direction of deformation (normal to surface) is easy to define automatically (a user should only be be able to flip this direction manually).