Replace SectorModel inputs, outputs with metadata.Spec
Replace SectorModel inputs, outputs with metadata.Spec

- reduce SectorModelBuilder responsibility to focus on loading
SectorModel class implementations from modules specified on the
filesystem. Defer construction to from_dict classmethod.

tomalrussell/smif,nismod/smif,willu47/smif,willu47/smif,tomalrussell/smif,tomalrussell/smif,nismod/smif,tomalrussell/smif,nismod/smif,willu47/smif,nismod/smif,willu47/smif