Time step future ideas

[sjboeing]

• Take into account moist physics in calculation of BV frequency
• Take into account surface fluxes to set the time step (e.g. for a neutrally stratified case with surface fluxes).

[matt-frey]

• Maybe make use of already computed derivatives of buoyancy (see Extract buoyancy gradient calculation #253)

[matt-frey]

This issue is related to #451 and #232.