Make Dataset a Context Manager.

[fzimmermann89]

We would like to be able to use the Dataset class in a with statement, i.e.

with Dataset(...) as ds:
   dowork

instead of

ds=Dataset()
dowork
ds.close() # we tend to forget this line..

This is a simple 4 line change to enable this, similar to how it is done for the File class

[fzimmermann89]

Any opinions on this?

Best regards from Berlin,
Felix

[dchansen]

Sorry for not catching this earlier. It's a good addition.