refactor code

[veenstrajelmer]

Limit public functions and simplify example scripts:

• move data retrieve to functions in data_retrieve.py and return ds (download of netcdf file is not present, open if it is present). Something like kw.data.timeseries("HOEKVHLD") and kw.data.extremes("HOEKVHLD")
• put default filenames in dict so their strings are not duplicated
• Many data_retrieve functions have station_list instead of station argument, do for all functions or not? >> no retrieve/read meas is per station, rest is for entire station_list
• move all analysis functions to data_analysis.py and call these functions from separate example script. Also include duplicate ext finding function from Incorrect extreme values for BERGSDSWT and others Rijkswaterstaat/wm-ws-dl#43. Execute per station and call in station-loop in example script.
• solve 5 remaining TODO in kw.data_analysis.plot_measurements() script. They are related to aggers and monthly/yearly averages for ts and ext. Maybe first make hwlwconvert modular so we can also apply it to an xarray.Dataset
• prevent exposure of private stats functions, they might be exposed via __all__ now
• solve TODO kw.data_analysis.plot_measurements() script about timezones >> maybe Add timezone to ddlpy.dataframe_to_xarray() Deltares/ddlpy#105 >> by always converting ds to df
• maybe convert netcdf back to ddlpy-like dataframe in kw.read_measurements(), but how to handle extreme types?
• convert kw.derive_statistics() from ds to df
• move tidal indicators to separate module, now in slotgemiddelden.py
• fix "TypeError: Cannot compare tz-naive and tz-aware timestamps" when running kwk_process.py
• check number of stations in locations df only once in data_retrieval.py, so move this check to a function
• kw.read_measurements() now returns None if no meas netcdf available, better alternative? Also returning none if no meas/loc available on ddlpy? Make this consistent or raise specific error that can be catched >> fine for now, maybe iterate later
• add plot_stations function #56

Follow up issue: #32