[data access] changed checking of index in position to index for Set Dim

so far, method raised an exception as soon as there were no labels.
[data access] removed unchecked addition of 1 to the count

and removed unnecessary code,
[data access] changed tests according to count changes
[data access] change get offset and count for tags as for multiTags
[data access] adapt tests to changes of offset and count estimation
Merge pull request #416 from jgrewe/data_access

Data access

LGTM