Support where for "union" and "forEach"

[niquola]

Support for where expressions for union and forEach may improve the readability.

Before:

{
  "name": "glucose_molar_concentration",
  "from": "Observation.where(code.coding.exists(system='http://loinc.org' and code='2339-0' or code='15074-8'))",
  "constants": [{"name": "glucose_to_molar", "value": 0.0555555556}],
  "select": [
    {
      "union": [{
        "from": "Observation.where(code.coding.exists(system='http://loinc.org' and code='2339-0'))",
        "select": [{"name": "g", "expr": "valueQuantity.value * %glucose_to_molar"}]
      }, {
        "from": "Observation.where(code.coding.exists(system='http://loinc.org' and code='15074-8'))",
        "select": [{"name": "g", "expr": "valueQuantity.value"}]
      }]
    }
  ]
}

After:

{
  "name": "glucose_molar_concentration",
  "from": "Observation",
  "where": "code.coding.exists(system='http://loinc.org' and code='2339-0' or code='15074-8')",
  "constants": [{"name": "glucose_to_molar", "value": 0.0555555556}],
  "select": [
    {
      "union": [{
        "from": "Observation",
        "select": [{"name": "g", "expr": "valueQuantity.value * %glucose_to_molar"}],
        "where":  "code.coding.exists(system='http://loinc.org' and code='2339-0')"
      }, {
        "from": "Observation",
        "where": "code.coding.exists(system='http://loinc.org' and code='15074-8')",
        "select": [{"name": "g", "expr": "valueQuantity.value"}]
      }]
    }

  ]
}

In this example from section in union can be skipped