Refactor set_viscous_BBL channel drag calculations

  Refactored set_viscous_BBL to separate out the routines setting the open
interface lengths used for the channel drag, shortening a 1070 line long routine
to 915 lines and reducing the scope of a number of temporary variables.  A
number of logical branch points have been moved outside of the innermost do
loops.  This refactoring will also make it easier to provide alternatives to
some of the solvers that do not use the trigonometric functions to solve for the
roots of a cubic expression and avoiding the issues noted at
NOAA-GFDL/issues/483.  All answers are bitwise identical and public
interfaces are unchanged.