Measure Resources should assume 'Boolean' based if not specified

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Description

Measure resources have the ability to specify on what basis they will be evaluated as for population counts in a Measure Report.

Specification defined this with the following extension: http://hl7.org/fhir/us/cqfmeasures/StructureDefinition/cqfm-populationBasis

When this extension is specified on a Measure resource it communicates whether it is a "boolean" based measure or a resource based measure like "encounter".

Issue

When the Measure does not specify the populationBasis of the measure, the MeasureEvaluator.class assumes it is a "resource" based Measure. The default behavior should assume "boolean" based populations if the extension is not populated to communicate otherwise.

This could cause unforeseen behavior in population counts as a result, where resources are counted instead of subjects. Or cause exclusion logic to incorrectly remove counts from other populations.