added phase material configuration for buffer layers in free surface simuations

examples/ConfigFiles/Phase_Isotropic_FreeSurface.config

@@ -0,0 +1,25 @@
+[Air]
+
+## Isotropic Material model to simulate free surfaces ##
+## For more information see paper Maiti+Eisenlohr2018, Scripta Materialia, 
+## "Fourier-based spectral method solution to finite strain crystal plasticity with free surfaces"
+
+elasticity              hooke
+plasticity              isotropic
+
+/dilatation/
+
+(output)                flowstress
+(output)                strainrate
+
+lattice_structure       isotropic
+c11                     110.9e9
+c12                     0.0
+taylorfactor            3
+tau0                    31e6
+gdot0                   0.001
+n                       20
+h0                      75e6
+tausat                  63e6
+w0                      2.25
+atol_resistance         1