Merge branch 'atmosphere/openacc_dyn_tend' into develop (PR #1308)

This merge adds OpenACC directives to the atm_compute_dyn_tend_work routine to
enable its execution on GPUs. As part of this work, time-invariant fields
required by the atm_compute_dyn_tend_work routine are copied to the device in
the mpas_atm_dynamics_init routine and deleted from the device in the
mpas_atm_dynamics_finalize routine.

The timing for data movement between host and device in the
atm_compute_dyn_tend_work routine is captured by a new timer,
atm_compute_dyn_tend_work [ACC_data_xfer].

* atmosphere/openacc_dyn_tend:
  Enable GPU execution of atm_compute_dyn_tend_work routine via OpenACC directives

Author: mgduda