minor change
JacobainD2 and JacobianD3 commented out
JacobainD2 and JacobianD3 added
psi_lam, internal force calculation modified, etc.
some minor editing
phi_lam changed to psi_lam (consistent w/ derivations), and other minor changes
dimensions for B1, B3, B4 and phi_lam corrected
dimensions for B1 and B3 corrected
n_str error corrected
initial check-in of solid constitutive model for mixture theory formulation