Change conditionPrincipleDiagnosis to conditionRoleIsBilling

[cragun47]

The FHIR specification advises that "the advice was to define Principal Diagnosis by constraining Encounter.diagnosis.role to "Billing Diagnosis" and requiring Encounter.diagnosis.rank =1."
Source: https://jira.hl7.org/browse/FHIR-15944

The model and processing should reflect this guidance.

• change Condition/model from three effectively boolean values ( conditionPrincipalDiagnosis
  conditionRoleIsAdmitting &
  conditionRoleIsChiefComplaint) to a single conditionDiagnosisUse, which has possible values: AD, DD, CC, CM, pre-op, post-op, and billing. (See https://terminology.hl7.org/3.1.0/CodeSystem-diagnosis-role.html)
• Add display names for these values in fhir_constants.EncounterResource
• Create processing in Condition/ffhirs based on the conditionDiagnosisUse
• Remove decoration of use code + Id. Use the straight ID

[cragun47]

Completed in PR: #17

[klwhaley]

Included in release 1.1.0 https://github.com/LinuxForHealth/CsvToFHIR/releases/tag/1.1.0