The P-F formulation seems to work just as fine as the S-E formulation; so time to start removing S-E implementation and clean everything up

Added export of First Piola-Kirchhoff stress tensor - exported as a general tensor in vtk.
Removed computeNLBMatrixAt from all standard elements but moved some of the element implementations to the corresponding derived gradient based element instead (for backward compatability).

Also removed/renamed computeBFmatrixAt to computeBHmatrixAt since B*a will give the displacement gradient H, not the deformation gradient F.
Cleanup of the code used for large deformation analysis and removed the implementation based on S and E. Now the code only has one large deformation implementation which is based on P and F.