Add the new module MOM_dynamics_split_RK2b and calls to the routines in this
module to MOM.F90.  These calls are exercised when the run time parameter
SPLIT_RK2B is true.  For now, all answers are bitwise identical when this new
code is being exercised, but there is a new module with multiple public
interfaces and a new entry in many MOM_parameter_doc files.

  Revised step_MOM_dyn_split_RK2b to use the time-filtered velocities as
arguments and reconstruct the instantaneous velocities with the proper phase
from the barotropic solver from the saved increments du_av_inst and dv_av_inst.
As a part of this change, the continuity solver is used to find the thickness
fluxes used in the predictor step Coriolis terms, and the horizontal viscous
accelerations are calculated for both the predictor and corrector steps, thereby
avoiding the need to vertically remap any 3-d fields apart from a single pair of
velocity components.

  The run-time option STORE_CORIOLIS_ACCEL is no longer used when SPLIT_RK2B =
True.  FPMIX was also disabled in this mode due to unresolved dimensional
inconsistency errors in that code.

  Additionally, the time-filtered velocities are now used instead of the
instantaneous velocities in the second call to radiation_open_bdry_conds(),
which should improve the performance of several of the Orlanski-type open
boundary conditions.

  The 2-d fields du_av_inst and dv_av_inst were added to the restart file, while
the 3-d fields u2, v2, diffu and diffv and either CAu and CAv or uh, vh and h2
are all removed from the restart files.  Remap_dyn_split_RK2b_aux_vars() now has
nothing to do, and it should probably be eliminated altogether in a subsequent
commit.

  All answers are changed when SPLIT_RK2B = True, and there are changes to the
contents of the restart files.

  This commit includes further revisions to MOM_dynamics_split_RK2b that avoid
some unnecessary calculations and group some of the halo updates into fewer
group passes.  All answers are bitwise identical.

  Corrected the description of the runtime parameter SPLIT_RK2B that will appear
in the MOM_parameter_doc files and in the doxygen descriptions of the MOM module
to better reflect what was ultimately being done with this new scheme.  All
answers are bitwise identical, but there are changes to the (newly added)
contents of some MOM_parameter_doc files.