Reformulate viscoelastic models to use strain rate rather than strain

[baagaard-usgs]

PyLith currently uses a strain-based formulation for viscoelastic models. Reformulating the time stepping using the PETSc TS object allows a more natural formulation using strain rate. All of the viscoelastic models (linear Maxwell, generalized Maxwell, and power-law) should be reformulated to use strain rate. This should allow the PETSc TS black-box adaptive time stepping algorithms to work for these materials.