feat: Measurement2D to represent 2D surface measurements (#54)

### Briefly, what does this PR introduce?
This adds a new datatype, `Measurement2D`, intended primarily but not
exclusively for use in Acts track finding. Therefore it tends to mirror
what we are already doing. The datatype stores two-dimensional (+time)
constraints on a surface with identifier.


There is additional motivation and discussion at
https://docs.google.com/presentation/d/1ee_iRAyeUao7PwkmFnapX7cASAp4rm8QnQ_moidYDnk/edit,
but changes may be applied here without updating the slides.

### 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
- [x ] Changes have been communicated to collaborators

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

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

---------

Co-authored-by: ShujieL <[email protected]>

edm4eic.yaml

@@ -336,8 +336,22 @@ datatypes:
       - float             edep              // Energy deposit in this hit [GeV]
       - float             edepError         // Error on the energy deposit [GeV]
 
+  edm4eic::Measurement2D:
+    Description: "2D measurement (on an arbitrary surface)"
+    Author: "W. Deconinck"
+    Members:
+      - uint64_t          surface           // Surface for bound coordinates (geometryID)
+      - edm4hep::Vector2f loc               // 2D location on surface
+      - float             time              // Measurement time
+      - edm4eic::Cov3f    covariance        // Covariance on location and time
+    VectorMembers:
+      - float             weights           // Weight for each of the hits, mirrors hits array
+    OneToManyRelations:
+      - edm4eic::TrackerHit hits            // Hits in this measurement (single or clustered)
+
+
   edm4eic::Trajectory:
-    Description: "Raw trajectory from the tracking algorithm"
+    Description: "Raw trajectory from the tracking algorithm. What is called hit here is 2d measurement indeed."
     Author: "S. Joosten, S. Li"
     Members:
       - uint32_t          type              // 0 (does not have good track fit), 1 (has good track fit)
@@ -353,8 +367,8 @@ datatypes:
       - float             outlierChi2       // Chi2 for each of the outliers
     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
+      - edm4eic::Measurement2D  measurementHits // Measurement hits used in this trajectory
+      - edm4eic::Measurement2D  outlierHits     // Outlier hits not used in this trajectory
 
   edm4eic::TrackParameters:
     Description: "ACTS Bound Track parameters"