Added method to reduce a full strain matrix to vector form. Maybe it should be placed under structural elements instead.
Added support for storing the deformation gradient in the material status for all structural elments.
Belonged to the previous commit
Modifications to the structure of large deformation analyses. Only works for full 3d Mat mode for now.

Have removed the need for element to implement NLBmatrix but the need to implement the BFmatrix
Added Deformations gradient as an IST for export