Efficiency improvements for CAM physics

[nusbaume]

This issue has been created to be a location where people can list possible efficiency modifications to various physics schemes that currently can't be applied due to the need to maintain bit-for-bit (b4b) reproducibility with the results from CAM.

Once all of the various schemes needed to reproduce CAM have been converted and verified, then these efficiency improvements can be applied.

To start, in apply_heating_rate_run in utilities/physics_tendency_updaters.F90, instead of doing:

 do klev = 1, nz
    temp(:, klev) = temp(:, klev) + (heating_rate(:, klev) * dt / cpair)
    dTdt_total(:, klev) = dTdt_total(:, klev) + (heating_rate(:, klev) / cpair)
 end do

One could do:

 cpair_inv = 1._kind_phys/cpair
 do klev = 1, nz
    temp(:, klev) = temp(:, klev) + (heating_rate(:, klev) * dt * cpair_inv)
    dTdt_total(:, klev) = dTdt_total(:, klev) + (heating_rate(:, klev) * cpair_inv)
 end do

which would reduce the number of (relatively more expensive) division calculations that are performed.

Feel free to add to this issue if any other efficiency improvements that could produce round-off changes are found.