Improve performance in creating groups
Improve performance in creating groups

Previously, we would return a group by calling __getitem__ after
creating it.
This would result in opening and reading the exdir.yaml file multiple
times.
This commit returns a group immediately after creation.
After all, we know the contents of exdir.yaml at this point.

CINPLA/exdir,CINPLA/exdir,CINPLA/exdir