precision and robustness of likelihood scans

the likelihood scans provided by the framework could use some improvements w.r.t. robustness of the result against starting values, malformed numerics in the likelihood and somesuch.

currently, the TSStatisticsCalculator has no interface to the RooFit offsetting machinery. This is required to improve the accuracy of likelihood scans beyond the machine limitations.

the TSLikelihoodScanner does not perform any sanity checks on the result and does not provide a lot of options for debugging broken likelihood scans