The Second Derivative Midpoint Formula requires we supply 3 points that are evenly
spaced apart, and that our target equals the x-components of the midpoint.
Example: differentiate(2, function($x) {return $x**2;}, 0, 4 ,3) will produce
a set of arrays by evaluating the callback at 3 evenly spaced points
between 0 and 4. Then, this array will be used in our approximation.
Second Derivative Midpoint Formula:
1 h²
f″(x₀) = - [f(x₀-h) - 2f(x₀) + f(x₀+h)] - - f⁽⁴⁾(ζ)
h² 12
where ζ lies between x₀ - h and x₀ + h