Converted the unit property, in the Agency Model, into its own Unit Model.
See issue here

Completed

• Created new Unit Model
• Established Unit as a child of agency
• Established an association between Unit and Employment object
• Updated Employment Model's merge_employment_records to account for Unit model
• Updated agencies and officers endpoints to reflect change to merge_employment_records
• Updated schema to include Unit model

To do next

• Create endpoint for Unit
• Create tests for Unit