Draft: Streamline data model

TODO:

• phi convention documentation
• Maybe introduce TrackerHitInfo etc utility classes for links
• Time resolution in tracker hit
• Use 4-vector + 4D diagonal covariance for tracker hit
• Remove redundant "polar" components - could be moved to Info utility class if useful
• Remove most of the ExtraCode for explicit constructors - this should either be a main Podio flag, but won't be necessary in the presence of the Info utility classes
• Rename RICHCluster to RingImage or similar
• Simplify some of the ExtraCode to make yams file look less scary
• Better name for the ImagingXXX classes
• Add momentum to ReconstructedParticle
• Add type to ECAL cluster
• Add weight to ReconstructedParticle