Include object name in UserWarning "Value with data type x is being converted to y"

[lvsltz]

Follow up on #298

Now I do get a warning like this one on write... but have no clue where it's coming from.
I think the warning message should also contain the object to which it refers.

./miniconda3/envs/nwb_latest/lib/python3.7/site-packages/hdmf/build/objectmapper.py:146:
UserWarning: Value with data type int64 is being converted to data type uint64
(min specification: uint32).
  % (g.name, ret_type.name, s.name))

[rly]

@lvsltz I made a PR to fix this issue. #311

The culprit of your situation is likely the sweep number, which is one of the few types that are specified as a uint. You can wrap your integer with np.uint to fix that. Perhaps pynwb.icephys.PatchClampSeries should be changed to cast the sweep number to uint if it can tell that the type is not a uint.