Efficiency improvements for CAM physics

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](https://github.com/ESCOMP/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.