BREAKING CHANGE: source now refers to a Source object, the FE that
created the Feature is now referred to by feature_extractor; path is
dropped

This should be more consistent, plus we needed a reference to the
original Source kept around anyway -- path worked but a Source object is
more consistent and explicit about intent

This lets us serialise properly

associated TODO: make precision configurable

This composes an Interval with some ancillary data:
 - source (what feature extractor did we get this from)
 - path (where is it now on the FS)
 - score (for ranking)

TODO double check we aren't overloading the word 'source' to have two
meanings

These are useful for changing Intervals (eg consolitation)