Refactor so we use @XmlSeeAlso to give track classes to JAXB. Instead of keeping list of known classes for marshalling/unmarshalling, we just try and if it fails we try the superclass recursively.

Use @SubtlyImportant annotation instead of comments, indicating methods which look like the aren't used but actually are (reflectively by JAXB)
Remove unused DataSourceTrack.normalize.

Add getter to call through to CoverageDataSource (add getter there too).
Add test for loading tdf through a session