Eliminate OBC loops in btloop_update_u

  Added code to avoid a separate OBC block within btloop_update_u and
btloop_update_v.  This was done by adding static CS%OBCmask_[uv] arrays that are
1 except at OBC points where they are 0, and using them to set BT_force_[uv],
IdxCu. IdyCv, f_4_u and f_4_v to Conversely bt_rem_u and bt_rem_v are being set
to 1 at OBC points.  Because all the changes occur by masking 2-d arrays during
the barotropic setup, there should be a net increase in model speed (if
anything).

  Also expanded the comments describing the nature and purpose of the Rayleigh_u
and Rayleigh_v arrays. and added commented out code that might use a new
answer date to avoid some unnecessary extra divisions in btstep_find_Cor.
All answers are bitwise identical and no public interfaces are changed.

Author: Hallberg-NOAA