Switch vignettes to using standalone data

[dshemetov]

Switch to using epidatasets data (if possible), otherwise add data to package.

Unclear on scope of some datasets.

[dsweber2]

Partner issue to cmu-delphi/epiprocess#365, copying a comment from Logan there:

We should not query the API in vignettes, so that users with firewall or API key problems or warnings do not have trouble trying to run code from the vignettes. The data used in the vignettes may already be packaged in objects in either epidatasets, epipredict, and/or epiprocess. If some is missing, it should be added to epidatasets, so we don't bloat epiprocess or epipredict.