Update constitutive_2d.f90

simus/constitutive_2d.f90

+!> @author
+!> Mattia de' Michieli Vitturi
+!> \date 15/08/2011
+!>
+!> Modification :: add friction weakening law
+!> @Author T. Frasson & A. Lucas
+!> \data 15/07/2020
+
+
 !********************************************************************************
 !> \brief Constitutive equations
 !********************************************************************************