Create helper to calculate potential temperature from exner

[jimmielin]

During review of #370, @nusbaume originally wrote:

Can we add a note in the "future work" issue to create a utility function that calculates the potential temperature based on the Exner function? That would not only be helpful here, but could also allow us to eventually get rid of the calc_exner_run physics scheme in utilities/state_converters.F90 as well.

This code is currently in schemes/vertical_diffusion/vertical_diffusion_interstitials.F90:

    ! Calculate potential temperature [K]
    th(:ncol,:) = t(:ncol,:) * exner(:ncol,:)