Add support for converting AGE_AT to patient birthdate

### Notes ###
- added handling of <code>AGE AT</code> during handling of <code>functionalOp</code>
  - attempt to use existing <code>birthdate</code> QDM element, otherwise create one using static data
  - create a <code>birthdate</code> reference and fix ordering on the precondition entry
  - convert the <code>functionalOp</code> to a <code>relationalOp</code> for proper temporal reference handling
- added unit tests and associated fixture data