add option of median statistics

[adokter]

During broad-front nocturnal migration there is often little spatial structure in the aerial migrants, and calculating median bird densities might be preferred over mean densities, because outliers are rejected automatically (like ground echoes of wind turbines). See also private email discussion 24-Sep-2020 Robb Diehl and KAMA20190901_084155_V06 case with wind turbine clutter