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)