[FEM] Add Temperature as a new plotType

Our application extensively uses FEM-Eyeshot in PostProcessing mode.

We have developed readers to import VTK files containing mesh and point/cell data fields.

We are currently able to plot most of them (displacement, stress, strain, and other tensor fields).

However, for thermo-mechanical analysis, there is also the scalar temperature field.

This field can be output either on points or cells (generally as a constant value within the element).

As a first step, you can simply add a new double property, Temperature, to both Node and Element, and handle the appropriate display accordingly.

thank you !