Models should be namespaced

[pelargir]

This gem defines dozens of models (most of them subclasses of Entry) but fails to namespace them within the HealthDataStandards. This means that my own application's Procedure, Provider, etc. classes get stepped on as soon as I import the gem.

+1 for properly namespacing these models.

[dillonwelch]

Created https://github.com/projectcypress/health-data-standards/pull/447 as a compromise attempt at this