Series approximation / derivative

[dirkwhoffmann]

The current code only approximates dercn, but is missing out derzn.

    // Perform series approximation if applicable
    if (ref.skipped) {

        dn = approximator.evaluate(point, d0, iteration);
        dn.reduce();
        dercn = approximator.evaluateDerivate(point, d0, iteration);
        dercn.reduce();
    }