Fixes Issue #190: add center id to MicroPhysiologyCenter model
Fixes Issue #190: Add center_id to AssayRun model
Fixes Issue #190: add center id to MicroPhysiologyCenter model
Fixes Issue #190: Add center_id to AssayRun model
Removed extra import
Rebase on Master to avoid merge conflict
Added spaces for PEP compliance; removed uneeded import
Merge pull request #192 from UPDDI/Issue-#190

Fixes Issue #190: Add center_id