$care-gaps functionality

[JPercival]

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https://github.com/hapifhir/hapi-fhir/wiki/Getting-Help

Describe the issue
The DaVinci DEQM IG specifies an operation called $care-gaps that extends Measure $evaluate-measure to produce reports that show any gaps in the care plan for a given subject relative to the clinical best practices encoded in the Measure. We should add this functionality to the HAPI FHIR server.

Environment (please complete the following information):

• HAPI FHIR Version
• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]

Additional context
Add any other context about the problem here.