Resolve the Requirement to Always have an Explicit Patient Context Line in Queries

[alexanderkiel]

Currently a CQL query without the line context patient won't run. The reason is that than, CQL-to-ELM generated the following Expression Def:

{
  "name" : "Patient",
  "context" : "Population",
  "expression" : {
    "type" : "SingletonFrom",
    "operand" : {
      "dataType" : "{http://hl7.org/fhir}Patient",
      "type" : "Retrieve"
    }
  }
}

The problem here is that the context is set to Population. In population context, the retrieve returns all patients and the SingletonFrom expression will fail on more than one.