Account for accelerations in reaction forces

[btalamini]

The reaction force calculation in the solid mechanics module assumes that the accelerations are zero:

https://github.com/LLNL/serac/blob/ef413804eda27556cec2b456ac45b44c830b23c1/src/serac/physics/solid_mechanics.hpp#L669-L673

This is fine for quasi-statics, but for dynamics, we must pass the updated accelerations in the second slot. It's not clear how to get this quantity out of the ODE solvers at the moment.