PyDataArray: Add count parameter to readData/_read_data

Change DataSet.read_direct to supply that count parameter (empty
tuple)
PyDataArray: Add offset parameter to readData/_read_data

Adapt DataSet.read_direct for it.
PyDataArray: Add count parameter to writeData/_write_data

Adapt DataSet.write_data for that change.
PyDataArray: Add offset parameter to writeData/_write_data

Adapt DataSet.write_direct() for the change.
PyDataArray: Extract common code into array_ensure_shape...
DataSet: Add single integer based indexing for reading data
[test] DataSet: Add test for single integer indexing
DataSet: support single Ellipsis indexing
[test] DataSet: Add support for single indexing

Also one test for unsupported indexing to keep coverage 100%
Merge pull request #36 from gicmo/indexing0

DataSet: Add single int, single ellipsis indexing for reading

Cool