Merge branch 'atmosphere/acc_advance_acoustic_step' into develop (PR #1251)

This merge adds OpenACC directives so the atm_advance_acoustic_step_work routine
can execute on GPUs. Also included in this merge are a few small clean-up
modifications.

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

Invariant fields used in this routine are also copied to the device within
mpas_atm_dynamics_init and are deleted in mpas_atm_dynamics_finalize.

* atmosphere/acc_advance_acoustic_step:
  Initial OpenACC port of atm_advance_acoustic_step_work

Author: mgduda