Split western and eastern OBC loops

  Split the loops applying western and eastern open boundary conditions in
apply_u_velocity_OBCs and southern and northern OBCs in apply_v_velocity_OBCs.
These usually cover very different loop ranges, and often there is only one of
the pair of OBCs on any processor, so it should be more efficient to only apply
the OBCs on within the loop ranges where they apply (which can be determined
during initialization).  The code still works for arbitrarily located OBC
segments, and all answers are bitwise identical.

Author: Hallberg-NOAA