Endpoint references

Endpoint references have been discussed at multiple occassions.
- During training courses and other USDM discussions
- With ARS team
- During scoping of phase 1 for USDM

Although tagging is available for endpoints it would be helpful for the industry to have a more specific referencing to
- timing
- assessments (in the form of Biomedical Concepts)

Each endpoint is basically a combination of a timing (with corresponding allowed window related to the main timeline) and an assessment. By clearly referring to them, it makes downstream automation and analysis much more feasible and the approach will be consistent in the future. This information then also can feed into the ARS in a more optimal way.

For the USDM model this would imply only two additional references since all other information is already in:
- Endpoint to Timing class (plannedAt? / allowedTiming?)
- Endpoint to BiomedicalConcept class (appliesTo?)