Use displacement data for solid mechanics restarts
New solid mechanics is only thermoelastic right now (see #471 (closed)), so doesn't require any special data for restarts. Once viscoplasticity is implemented, there will be a state that needs to be stored and read in for restarting. However, thermoelastic solid mechanics does use the previous displacement solution as the initial guess at each time step. Without that displacement data, the first SM solve after a restart will take longer than subsequent solves. We should consider addressing this when restarts are implemented for viscoplasticity.