trig_bisection_test
  Use bisection to estimate a solution to the trig equation.

  F(1) = -0.459698 (negative)
  F(0) = 1 (positive)

  After 20 iterations:
  F(0.739085) = -1.07502e-07 (negative)
  F(0.739084) = 1.48858e-06 (positive)

  it <= itmax?  true
  |xn-xp| <= xtol?  true
  min(|f(xn)|,|f(xp)|) <= ftol?  true