More robust ridders
More robust ridders

1. Following code is incorrect for approximate comparison to zero.

> x ~= 0 ≡ abs (x - 0) <= abs (tol * x) ≡ abs x <= abs (tol * x)
> x ~= 0 ≡ True if tol > 1

So direct comparison to 0 is used.

2. Add safeguards. If root is bracketed within 1 ulp return root
regardless of requested accuracy.

3. If Ridder's approximation coincide with with one of old
   bounds revert to bisection

4. Document that absolute error esimate is used.

With these changes in many cases 0 could be used as requested
Accuracy. Funcion will find root with 1 ulp precision. It still
may fail to locate root especially near 0.

Affects #14

00tau/statistics,00tau/statistics