GroupedList: Enable grouping by a function result. Example: Want to grou...

GroupedList: 

Enable a functionality that worked in SIlverstripe 2.4:
Grouping by a function result. 

Example: Want to group by the Year part of a Date field, build a function Year() that extracts the year from the date, then use GroupedBy(Year) in the template.

(Maybe my fix is a bit dirty, but it works well in my projects)