pub fn cotd(degrees: FLOAT) -> FLOAT
Returns the cotangent of the argument given in degrees
assert_approx_eq(cotd(45.0), 1.0, 1e-10);
assert_approx_eq(cotd(90.0), 0.0, 1e-10);
assert_approx_eq(cotd(135.0), -1.0, 1e-10);