Optionally use percentiles in db_access::get_date_growth_rate

[kevinkleinQC]

As of now the date growth is defined by new_date_range / old_date_range - 1, each range being defined by the difference between maximum and minimum therein.

Yet, using minima and maxima is very sensitive to outliers. Instead, one might favour the more robust approach of using the pth and 100-pth percentiles.

This could be an optional feature of db_access.get_date_growth_rate.

Suggestion by Stefan Sorg.