Enable GPU execution of MPAS-Atmosphere initialization of coupled diagnostic fields via OpenACC

[gdicker1]

This PR enables GPU calculation of certain diagnostic fields during the initialization phase by adding OpenACC directives to the atm_init_coupled_diagnostics routine.

Timing information for the OpenACC data transfers in this routine is captured in the log file by a new timer: atm_init_coupled_diagnostics [ACC_data_xfer].

Since this routine is called during the initialization phase, before mpas_atm_dynamics_init, no modifications were made to the mpas_atm_dynamics_{init,finalize} routines to copy in or delete the invariant fields used in this routine.

[gdicker1]

NOTE: the changes in this PR do slightly change answers due to the calculations of exner and exner_base. If the loop to calculate them (lines 5974 to 5982 in this PR) is instead done on the CPU and appropriate update directives are added, there is no difference in the answers.

I suspect the answer difference is likely due to a difference in how the CPU and GPU handle the calculation of a float raised to a float.

[mgduda]

@gdicker1 I agree that the bitwise differences seem to be due to the exner calculation. If I add this block of code directly below the exner calculation on the device, I get bitwise identical results before and after merging this PR locally:

      !$acc update host(rtheta_p, rtheta_base)
      do iCell=cellStart,cellEnd
         do k=1,nVertLevels
            exner(k,iCell) = (zz(k,iCell) * (rgas/p0) * (rtheta_p(k,iCell) + rtheta_base(k,iCell)))**rcv
            exner_base(k,iCell) = (zz(k,iCell) * (rgas/p0) * (rtheta_base(k,iCell)))**rcv
         end do
      end do
      !$acc update device(exner, exner_base)

[gdicker1]

Fixup commit 2f95243 addresses this comment I made.

[gdicker1]

This is ready to merge!