correction and split agreement into 2 functions

[MaelleTtrt]

I split the agreement into 2 functions
1-count_detections_within_timeframe that returns a dataframe containing counts of detections within a timeframe
2 - plot the R²
I also corrected the function agreement so that the last bin encompassed properly the dataset

    bins = date_range(
        start=df["start_datetime"].min().floor(bin_size),
        end=df["end_datetime"].max().ceil(bin_size),
        freq=freq,
    )