Update the default measurement of interest to be Hemoglobin A1c.

[deflaux]

The primary reason to do this was to use a measure found in both the real data and the synthetic data, but I also think its a better default measure to use.

• See what the plots look like here.
• See the successful smoke test run here.

Unfortunately we don't have automated testing configured for the code in this repository yet so we set up this checklist as an automatic reminder:

• [X] Ensure that the smoke tests pass using the current (or upcoming) CDR
• [X] Update documentation relevant to this pull request

Questions? See CONTRIBUTING.md or file an issue so that we can get it documented!