Fix extension contexts and move location of repo

[lmckenzi]

1. A number of the extensions defined in this IG have a scope of 'Element', which is broader than the obviously intended scope of the extensions.  Please assert contexts that are narrower and reflect where the extensions are actually expected to be used.
2. This IG claims to be owned by the HL7 Public Health work group. If that's the case, then the content for the IG must be developed and maintained under the HL7 github organization, not under a different Git project. (An IG proposal will also need to be approved by the work group and the FMG.)

[saulakravitz]

Hi Lloyd,
This repo is a playground/sandbox for our efforts to harmonize the vital records (VRDR, BFDR, VRCL) and MDI IGs. It is not something that will be brought to ballot. The content that is being brought to ballot is under the HL7 organization. We have already addressed the extension context issues there.

Thx, Saul