Let Container.write_dataframe accept a dict

[danielballan]

It is convenient that Container.write_array accepts a list, like:

$ tiled serve catalog --temp --api-key=secret

In [1]: from tiled.client import from_uri

In [2]: c = from_uri('http://localhost:8000', api_key='secret')

In [3]: c
Out[3]: <Container {}>

In [4]: c.write_array([1,2,3], key='x')
Out[4]: <ArrayClient shape=(3,) chunks=((3,),) dtype=int64>

It would be convenient if

c.write_dataframe({'a': [1,2,3], 'b':, [4,5,6]}, key='y')

were likewise acceptable. Currently, a pandas.DataFrame or dask.dataframe object must be passed.