PyFile: docstrings for File and its mixin
PyDataArray: textual description for data array
test_data_array: fix test with data_extent

array.data_extent = (200, 0) does not work anymore.
(200, ) works, I hope this is correct.
docstrings: move string definitions to *.cpp file

The old version was producing warnigs about unused variables.
PyDataArray: define stubs for binig docstrings
PyDataArray: documentation complete
Merge pull request #22 from stoewer/documentation

Documentation for DataArray

Awesome.