+Add find_L_open_concave_iterative

  Added the new routine find_L_open_concave_iterative to use iterative Newton's
method approaches with appropriate limits to solve the cubic equation for the
fractional open face lengths at interfaces that are used by the CHANNEL_DRAG
code.  These solutions are analogous to those given by the previous expressions
that are now in find_L_open_concave_trigonometric, and the two differ at close
to roundoff, but the new method is completely independent of the transcendental
function library, thereby addressing dev/gfdl MOM6 issue #483.  This new routine
is called when the new runtime parameter TRIG_CHANNEL_DRAG_WIDTHS is set to
false, but by default the previous answers are recovered.  By default all
answers are bitwise identical, but there is a new runtime parameter in some
MOM_parameter_doc files.