Split apply_velocity_OBCs into two routines

  Split apply_velocity_OBCs into apply_u_velocity_OBCs and apply_v_velocity_OBCs
to allow for shorter lists of arguments and more targeted routines.  Also moved
the calls to store_u_for_OBCs and store_v_for_OBCs to be just before the calls
to btloop_update_u and btloop_update_v as a step toward possibly combining these
routines.   All answers are bitwise identical and no public interfaces are
changed.

Author: Hallberg-NOAA