Move some parts of btstep() into subroutines

The short description of each routine is:
btstep_ubt_from_layer: Calculate the zonal and meridional velocity from the 3-D velocity.
btstep_find_Cor: Find the Coriolis force terms _zon and _mer.
btloop_setup: Set ubt, vbt, eta, find face areas, and set valid array size at the beginning of each barotropic loop.
btloop_setup_eta: A routine to set eta_pred and the running time integral of uhbt and vhbt.
btloop_setup_OBCs: Setup old or previous ubt, vbt, vbt, and vhbt for different OBC options before the next step in the btstep loop.
btloop_update_v:  Update meridional velocity.
btloop_update_u: Update zonal velocity.
btstep_layer_accel:  Calculate the zonal and meridional acceleration of each layer due to the barotropic calculation.

Author: Theresa Morrison