pass complete measurement series throughout code

[veenstrajelmer]

Troughout the KWK_process.py example script there are timeseries used with different start/stop times. Instead pass todate timeseries everywhere and make sure that some functions only use the last 10/19 years.

Todo:

• pass df_meas_todate and df_ext_12_todate to all functions and take last 10 years of timeseries if function requires this instead
• add test for new function
• also raise error/warning if e.g. 4 years is provided instead of 10 years? Maybe fine to hard-code expected number of years, but only if we make the mean value/time computation a modular function. This would require update some of the tests >> for now only added a warning if the resulting amount of years is less than requested.