Fixed segment_dat for newsamples == 0
Added __str__ for Data
Fixed bug in Data.__eq__ => implemented __ne__ as well
Better test for equality.
Added test for copy.
Revert "Return an empty array with the correct number of dimensions."

This reverts commit 5a45b0470ff9a14eebead06d4bd38a05f406c86f.
Empty Data => .data == np.array([]) (.size == 0)
Added __nonzero__ for easy checking if Data is empty.
Fixed docstrings for Data and Ringbuffer classes.