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`Transform`s are now `Operator`s and the special `Stream` type has been replaced by the data access operator.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
this operates on data access operators?  i thought there were data access operators, executors, and connectors between the two?

I don't feel strongly about this---it's just something we discussed yesterday