coh2d elem cohesive law tested

- coh2d elem debugged, coh law tested
- previous error: only 1 execution can be done after the if(...); this
caused the erroneous zeroing of fstat repeatedly at each increment
- mixed mode fracture toughness, avg of 2 shear toughness was erroneous
previously; now a linear interpolation btw the two

now both the failure criterion and the cohesive law works correctly
next step: populate all cohesive elements