UMAT Utilities

[TeroFrondelius]

Someone else proposed this earlier, I couldn't find it. Anyway, I found the list of UMAT Utilities:
– SINV will return the first and second invariants of a tensor.
– SPRINC will return the principal values of a tensor.
– SPRIND will return the principal values and directions of a tensor.
– ROTSIG will rotate a tensor with an orientation matrix.
– XIT will terminate an analysis and close all files associated with the analysis properly.

If we want to have full compatibility we will eventually need to support these Fortran subroutines.

Also #6 is a same thing.