General parameter `_elements` not supported through read interaction

[kennethmyhra]

Describe the bug
General parameter _elements is not supported through read interactions. It is only supported through search interactions.

To Reproduce
Steps to reproduce the behavior:

1. Execute read interaction: https://spark.incendi.no/fhir/Patient/example?_format=json&_elements=name,birthDate

Expected behavior
When applied to read interactions should return a resource with only the requested _elements included.

Spark version

• Version: 2.5, 3.0.1

Failed tests
ElementsSearchParameterTest (I_ELEMENTS_002)