Add support for labelling coordinates

[josevalim]

xarray can provide a lot of guidance here: http://xarray.pydata.org/en/stable/

It would also be interesting to see tensor backends that target libraries/formats such as pandas/apache arrow and see the changes that we would need to make to model to support those.

[josevalim]

After reading xarray, they actually use a separate datatype, called Dataset, for their Pandas equivalent, so we would probably need a separate data-type here too.

There is one potential interesting interplay here which is to allow Dataset in defn, but that will only be possible if all Dataset columns are valid tensor types. For example, if a dataset column is a string, then we obviously can't use it in defn. So my suggestion is to explore the dataset approach in a separate library and potentially make each column a tensor.