new class DataHandlerMixedSamplingWithFilterSingleStation that bundles common methods of the kz and fir filter when used as mixed sampling

kz filter has moved to a new location together with the new FIR filter class, first implementation of FIR data handler

new tests for testing helpers, corrected wrong usage of PyTestAllEqual and also refactored this method, /close #242

expanded input date range for mixed sampling data handler (will not expand target date range and therefore sampling date range, but reduced data loss)