Enhance the estimator API

[PhilippSchuette]

Right now the ArgumentEstimator implementations build the logarithmic derivative from the target function f and its derivative df themselves. For some functions it might be useful to be able to provide the logarithmic derivative df / f directly. This way one could e.g. simplify this derivative by hand or precompile it separately. There might even be use cases where only the logarithmic derivative is given...