Added method Tie_Point_Grid.calc_overall_stats() + tests to compute overall...

Added method Tie_Point_Grid.calc_overall_stats() + tests to compute overall statistics from all tie points found.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>
3 jobs for feature/add_stats in 4 minutes and 7 seconds (queued for 1 minute and 42 seconds)