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

Merge files from felix_issue267-create-wrf-chem-data-handler: mlair/helpers/geofunctions.py, mlair/helpers/helpers.py, test/test_helpers/test_geofunctions.py, test/test_helpers/test_helpers.py

renamed the IntelliO3 reference model and its tests, added newest version of netCDF4 to requirements