FEM: femmesh2mesh add disp scale factor

[fandaL]

Adds the scale factor of the deformed shape to the function femmesh2mesh, as discussed:
https://forum.freecadweb.org/viewtopic.php?p=638649#p638649

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[freecadci]

for feature branch PR_7715. Pipeline 686428208 was triggered at a6ec9ba. All CI branches and pipelines.