Fix transposition in logit
Remove I2 from pressure
Return I2 instead of C:C in analyze_params_u...
Return the pressure with the constitutive model

This doesn't rely on a closed-form expression for pressure, and
so should make more general loading doable.
Use generalized pressure and deformation gradient for all args
Re-added helper functions for arbitrary Fs
Since general pressure works, remove uniaxial
Fix typo in det1_3d
Remove superfluous pressure argument to test_mr_drucker
Remove deprecated IvI and CivCi from MR tstiff
Fixed some typos