Maybe move to another header.
Add consistency check on re-open of an existing group
implement
add structure function
add possibility to share permutation index datasets
Implement.
use newer field interface
Implement
use newer field interface
Make use of timestamps of the involved fields (whatever this means)
Define an exception class here to indicate the many reasons of failures here, instead of just returning a NullPtr.
should just return a pair<> here as return value, and rather use a fragment ID itself instead of a string
CHECK WHETHER PARAMETERS REALLY NEED TO BE COPIED HERE