Introduce simulated tempering for modular simulator

Simulated tempering is an extension of the expanded ensemble functionality. Simulated tempering is covered by the new free energy tests.

Simulated tempering updates the reference temperature. This requires the reference temperature update clients to be aware of the new algorithm.

This also changes the energy data object to write at free energy writing steps rather than energy calculation steps. This had no influence until now, as the two were generally identical. The current change aligns the modular simulator with the legacy implementation.

Refs #3417