Pluggable data sources for Metis

Metis now accepts data sources, executors, and their connecting parts (data access operators) from arbitrary locations.

`Transform`s are now `Operator`s and the special `Stream` type has been replaced by the data access operator.

You can look at this, but I still need to fix/run tests. It does work with both Python and Spark executors on some query I made up.

Note: happens to fix #336
rename sounds good!:)

On Wed, Dec 17, 2014 at 4:31 PM, Derek Schultz <notifications@github.com>
wrote:
>
> In metis/metis/core/query/operator.py
> <https://github.com/Locu/chronology/pull/340#discussion-diff-22005920>:
>
> >      del _dict['type']
> >
> > +    if typ == Operator.Type.DATA_ACCESS:
> > +      return DataAccess.parse(_dict)
> > +
> >      if 'stream' in _dict:
>
> Yeah, actually that is still how you flow data through the nested
> operators. I should rename it source.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Locu/chronology/pull/340/files#r22005920>.
>