Ongoing discussion about where to find mappings for terms as well as values

I checked [via_terminology_server.ipynb](https://github.com/cancerDHC/example-data/blob/issue-27-codable-concept-notebooks/via_terminology_server.ipynb) into this repo's `issue-27-codable-concept-notebooks` branch and created PR #28

I had two goals:

- reimplement the GDC to CRDC-H Diagnosis conversions in [GDC to CCDH conversion.ipynb](https://github.com/cancerDHC/example-data/blob/main/GDC%20to%20CCDH%20conversion.ipynb) to use codeable concepts instead of enumerations
- uncouple mapping knowledge from executable code by using the CCDH terminology server. I would say that's going well for body sites and cancer stage observation values, but not for cancer stage observation types. The conversion of the GDC cancer stage observation types is currently hardcode as a bunch of if statements. I don't see how to look those up with the terminology server.