'ARPALdf_Summary' returns many descriptive statistics summaring the data contained in a data frame
of class ARPALdf. Statistics are calculated at overall level (full sample), by station ID and by year.
For each variable are reported the basic positioning indices (min, max, mean, median, quantile) and
variability indices (range, standard deviation). Other reported statistics are the Pearson's linear correlation
by station and some graphical representation of the distribution (kernel density plot, histogram, boxplot).
In addition, the function returns useful data-quality information, such as gap length statistics (i.e. number of
missing observations for each variable by station and by year) and outlier detection tools
(e.g., Hampel filter and boxplot rule)