pk_flu_week_ix - calc max incid wk, fluweeks only
pk_flu_week_ix - calc max incid wk, fluweeks only

functions.py
- add peak_flu_week_index function
- calculate week of maximum incidence based on flu weeks
only
- classif_zOR_index and classif_zOR_processing functions
all reference this function now for the calculation of
the retrospective index

eclee25/flu-SDI-exploratory-age,eclee25/flu-SDI-exploratory-age