added condition 'contact is in group'

This adds a new condition, checking if a contact is in one of, all of, or none of a number of specified groups. It mirrors the “Contact has Tags” condition. A helper class “SetOps” is introduced, requiring that the arrays used represent sets with the set members stored in array keys (not values).

Both “Contact is in Groups” and “Contact has Tags” have a logical irregularity: If the no groups are set in the condition and the mode is “has all of these tags / is in all of these groups”, the condition returns false. From a (set) logical point of view, it should always return true. This is for pragmatic reasons. If the condition uses a number of tags/groups but at a later time, these tags/groups disappear, then it is probably not intended to run the associated action for all contacts when previously it had been restricted to some tags/groups.