[FEM] add new filter to display iso contours

[donovaly]

• very valuable to display for example electromagnetic field lines

This also works for 3D.

Here is a good file to play if anybody is interested:
MagneticHorseshoe.zip

Here is a screenshot how it looks in 2D:

Field lines of the electric potential of a magnetic horseshoe

The same horseshoe, but the magnetic field strength magnitude

and here in 3D:

Iso-surfaces of the principal stress in a cantilever beam that is bent. The contour coloring is turned off (pipeline provides this feature.).

The filter can of course be combined with other filters. Here an example in a warp filter:

[freecadci]

for feature branch PR_8462. Pipeline 775312299 was triggered at 429cfe1. All CI branches and pipelines.

[freecadci]

for feature branch PR_8462. Pipeline 775363919 was triggered at 58e5b49. All CI branches and pipelines.

[freecadci]

for feature branch PR_8462. Pipeline 776070860 was triggered at 2f7e12b. All CI branches and pipelines.