Bug fix: Transforming triangle normal to appropriate co-ordinate system.

- The triangle normal in TriangleMeshTriangleMeshDcdContact is now being
  transformed into the other triangle's co-ordinate system.
- Modified test case to expose the bug.
Change the variables names to better reflect them.

- Also includes one cpplint fix.
Change the TriangleMesh::getNormal to return a const Vector3d&.

- Currectly it returns a copy of the normal, whcih is not efficient.
Merge branch 'master' into fix/TriangleTriangleDcdContact
Merge branch 'fix/TriangleTriangleDcdContact' into 'master'

Bug fix: Transforming triangle normal to appropriate co-ordinate system.

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/774793