Steady state HPMR model can be made less expensive

[aprilnovak]

The HPMR microreactor steady state model currently includes a time derivative in the BISON heat conduction calculation. The problem, however, is steady state. This time derivative forces each BISON run to ramp up to a steady state using 50 time steps. By removing the time derivative, the overall cost of the HPMR simulation can be cut by ~50x, without affecting the results.