- Without this, one can see the tile boundaries in the uhml and vhml
  fields.

Optionally correct a bug due to a missing halo exchange.  This likely
isn't needed when compiled for nonsymmetric memory, but the added halo
exchange does no harm in such cases.  The pass_vector call could probably
be replaced with fill_symmetric_edges, except there is no subroutine
fill_vector_symmetric_edges_3d.

USE_CONT_THICKNESS is not yet widely used, so rather than preserving the old
(incorrect) solutions by default, this bug is corrected by default.  However,
the previous answers can be recovered by setting the new runtime parameter
USE_CONT_THICKNESS_BUG to true.  This parameter is only used (and logged)
when USE_CONT_THICKNESS set to true.

By default, this commit does change answers in symmetric memory cases with
USE_CONT_THICKNESS = True, and there is a new runtime parameter in such cases.

* Directional linear wave drag

This commit allows the linear wave drag to be applied to the u and
v components of the barotropic velocity separately.