The current RideFile type associates a unique interval number with each sample
point, meaning that intervals can't overlap. It also names intervals as
integers, not strings. So for now, XmlRideFile just orders intervals by their
order in the xml file and names them by their order in this list (starting
with zero, to match convention). It then associates each sample with the
lowest-named interval into which the sample falls. This strategy means that a
raw file exported to xml will have the same interval locations and names when
read back in as xml.
