Add a maximum change envelope column to statistics file

[robbibt]

The only change I'd suggest, to accomodate the impact-recovery type changes would be to have a value of the 'the coastline has moved by up to z metres at this location' where z is the range between maximum landward and maximum seaward. This should capture both monotonic and event-recovery type changes in a single metric and let users evaluate whether or not it's erosion, cyclical, step, gradual etc from the profile

Filter outliers before calculating change