added utils.R to drop dependencies with urbanR library, which will not be available when Nord3v2 discontinues

Code to calculate the values established by the regulations. It remains to develop the code to plot the maps.

df_obs_sub has been modified when we don't have the data for all dates, putting NA to have the same length as length_loop

slope > -20 considered for correction, otherwise NA's are set. Barnes statistics in a different script