Make Fem2DElementTriangle::computeRotation a reusable method

This method was setting up the variable m_initialRotation internally.
It should really return the matrix and the assignment should be done by the caller.
This can be useful for a potential co-rotational FEM triangle element that would derive from this class.
Merge branch 'feature/fem2DcomputeRotation' into 'master'

Make Fem2DElementTriangle::computeRotation a reusable method

+1: Gughan Sathyaseelan, Ryan Beasley, haizhouW

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/1157723