add apis to loop through all data record groups

[excaliburtb]

relatively simple request for once :)

the current set of apis allow one to retrieve a DataRecordGroup pointer via a name but we could use two more APIs get_num_data_record_groups and get_data_record_group_by_idx or something like that. it would allow one to loop over them.

[sharmeye]

Then for once we won't yell at you.