+Non-Boussinesq Flather open boundary conditions

  Get Flather open boundary conditions working properly in non-Boussinesq mode.
This includes calculating the column-average specific volume in
step_MOM_dyn_split_RK2 and passing it as a new argument to btstep.  Inside of
btstep, this is copied over into a wide halo array and then passed on to
set_up_BT_OBC and apply_velocity_OBCs, where it is used to determine the free
surface height or the vertical column extent (in [Z ~> m]) from eta for use in
the Flather radiation open boundary conditions.  In addition, there are several
places in MOM_barotropic related to the open boundary conditions where the usual
G%bathyT needed to be replaced with its wide-halo counterpart, CS%bathyT.  Also,
a test was added for massless OBC columns in apply_velocity_OBCs, which are then
assumed to be dry rather than dividing by zero.  A fatal error message that is
triggered in the case of Flather open boundary conditions in non-Boussiesq mode
was removed.  With this change, all Boussinesq answers are bitwise identical,
but non-Boussinesq cases with Flather open boundary conditions are now working
and giving answers that are qualitatively similar to the Boussinesq cases.