DaskDataFrame: Fix Chunks (#959)

For more than one chunk, the extent needs to add the offset
to slice the right size.

Author: ax3l

src/binding/python/openpmd_api/DaskDataFrame.py

@@ -20,7 +20,7 @@
 
 
 def read_chunk_to_df(species, chunk):
-    stride = np.s_[chunk.offset[0]:chunk.extent[0]]
+    stride = np.s_[chunk.offset[0]:chunk.offset[0]+chunk.extent[0]]
     return species.to_df(stride)