feat: updates to tracking structures (#42)

### Briefly, what does this PR introduce?
This makes a number of practice-inspired modifications:
- Trajectory can include multiple TrackParameters
- Track can include one Trajectory
- Track can include one Vertex

Since Track is not currently used, and since Trajectory expands the
number of TrackParameters, this is backwards compatible for practical
purposes.

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [x] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No.

### Does this PR change default behavior?
No.

edm4eic.yaml

@@ -340,9 +340,8 @@ datatypes:
     VectorMembers:
       - float             measurementChi2   // Chi2 for each of the measurements
       - float             outlierChi2       // Chi2 for each of the outliers
-    OneToOneRelations:
-      - edm4eic::TrackParameters trackParameters // Associated track parameters, if any
     OneToManyRelations:
+      - edm4eic::TrackParameters trackParameters // Associated track parameters, if any
       - edm4eic::TrackerHit  measurementHits // Measurement hits used in this trajectory
       - edm4eic::TrackerHit  outlierHits     // Outlier hits not used in this trajectory
 
@@ -375,8 +374,10 @@ datatypes:
       - float             time              // Track time at the vertex [ns]
       - float             timeError         // Error on the track vertex time
       - float             charge            // Particle charge
+    OneToOneRelations:
+      - edm4eic::Trajectory trajectory      // Trajectory of this track
+      - edm4eic::Vertex   vertex            // Track vertex of this track
     OneToManyRelations:
-      - edm4eic::TrackParameters parameters // Track fit parameters, the first entry (if present) is evaluated at the track head
       - edm4eic::TrackerHit trackerHits     // Hits that were used for this track
       - edm4eic::Track    tracks            // Tracks (segments) that have been combined to create this track