Merge explicit_namespace branch in to tufts_models release candidate

[mark-dce]

• [x] rebase branch
  - [ ] test branch on models created with default namespace version of MIRA
  - [ ] test branch on tufts fixtures
  - [ ] test branch on xml imports with default namespace models
  - [ ] test branch on xml imports with explicit namespace prefixes
• [x] rebase master into feature branch and make the explicit_namespace branch a release candidate