Replace it by distinguishing between "Loadable" and "Computable". This is useful because it's now possible to write computable terms that don't require any inputs (e.g. an `Always` filter or an `Everything` classifier).
Previously we were only filtering input nodes.
Added CompositeTerm and now we dispatch more generally on atomic
zipline.modelling.* -> zipline.pipeline.* zipline.data.ffc.loaders -> zipline.pipeline.loaders tests/modelling -> tests/pipeline
Scott Sanderson