570 bug on handling conversion from a choice type 2

[baseTwo]

Work in Progress for #570
Continuation for PR #571
This should be considered conceptual and used for discussion only at this stage

While developing I found a lot of conversions don't exist, for example in MATGlobalCommonFunctionsFHIR4.Patient.Normalize Interval

Could not resolve on or more case expressions for Choice type Choice<{http://hl7.org/fhir}dateTime, {http://hl7.org/fhir}Period, {http://hl7.org/fhir}Timing, {http://hl7.org/fhir}instant, {http://hl7.org/fhir}string, {http://hl7.org/fhir}Age, {http://hl7.org/fhir}Range>.
These conversions could not be resolved:
- Hl7.Fhir.Model.Period to Hl7.Fhir.Model.FhirDateTime
- Hl7.Fhir.Model.Timing to Hl7.Fhir.Model.FhirDateTime
- Hl7.Fhir.Model.Instant to Hl7.Fhir.Model.FhirDateTime
- Hl7.Fhir.Model.FhirString to Hl7.Fhir.Model.FhirDateTime
- Hl7.Fhir.Model.Age to Hl7.Fhir.Model.FhirDateTime
- Hl7.Fhir.Model.Range to Hl7.Fhir.Model.FhirDateTime

A setting was added in ExpressionBuilderOptions to either throw or ignore these cases. If it is ignored, the C# code will show in a comment which types were missing.

CqlDateTime g_ = f_ switch { null => null , CqlDateTime a => a/* , FhirDecimal => ??? */, _ => throw new System.Diagnostics.UnreachableException() };

[baseTwo]

Work continues in #608