Add interval_mean_absolute_error to Group Tags

[Gofven]

Create new variable "interval_mean_absolute_error" for GroupTags, display it for everyone

Assign the variable with following: For every combined_bet & outcome in a given tag: abs(sum(combined_bet) – sum(outcome))/N

• N is the number of predictions that had at least one bet

[Gofven]

See if it works now c: