Force inference does not work

[sophietheis]

Looking for where the problem is, but it is still a mystery...
It seems that the matrix g_gamma is construct correctly.

Maybe we should not perform a qr decomposition and we should use an other method ?

Found a small factor error, pos is the full lenght of the edge, and normally we should take into account an half length edge. But it does not really have an influence in the tension result.