Fix bug in membrane shape function + reduce equations
    m_membraneShapeFunctionsParameters(0, 2) = inv_2A * x2 - x1;
should be
    m_membraneShapeFunctionsParameters(0, 2) = inv_2A * (x2 - x1);
Fix Rotation transform + minor improvement

The element initial rotation R0 transform from local coordinates to global coordinates, so when transforming a Mass/Stiffness local matrix into a global matrix, it is:
Mglobal = R0.Mlocal.R0^T
we were doing the opposite.
Improve documentation and fix spacing
Merge branch 'fix/fem2dBugs' into 'master'

Fix 2 bugs in Fem2D

+1: Walter Schoen, gsathyaseelan, Harald Scheirich

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/814583