FEM integration by combining flamingo with Arch Axis

[luzpaz]

Ref: https://forum.freecadweb.org/viewtopic.php?f=8&t=17035#p134768
CC @berndhahnebach

[berndhahnebach]

the FEM analysis was the main motivation to introduce the structural axis for a Arch Structure. At the moment it is a bit quired in this regard but it should work.
bernd