Iterative BSSubtype of
Iterative bootstrap is needed when a non-local interpolation (such as cubic splines) is used.
In this case, setting the value of a node modifies the whole curve, invalidating previous nodes.
Therefore, we must go over the nodes a number of times until convergence is reached.
The specification details of this method can be optionally customized by supplying an object of type as value to the key Bootstrap Spec