Changed iif to work with the correct context

[Kasdejong]

Description

the FP iif function should now work with the correct focus.

Related issues

Resolves #2625

Testing

Extra iif tests.

[Kasdejong]

The expected (in hapi and fplab) behaviour seems to be strange here. It is also not well documented and it relies on reflection. For example, on fp-test-patient, where Patient.contained[0].name is an array of 2 names:

• "Patient.contained[0].name.iif(exists(), 'named', 'unnamed')" is expected to run exists() on the name array and return a single string.
• "Patient.contained[0].name.iif(use = 'official', given, 'not official')" is expected to run use = 'official' on every individual name element (as if there is a hidden select there), and so it returns an array of strings.

We cannot determine at compile time if name is enumerable, so instead, we would have to mark a method as a method which works on collections on or single elements, and add an overload to iif. I feel like this was not the intention, and I would like to discuss with eg @brianpos about what truly is expected of us.

It is worth noting that we pass the official tests by accident! "Patient.contained[0].name.iif(exists(), 'named', 'unnamed')" throws an error (iif not supported on lists), but still manages to return the correct result, since it only runs on the first element on the list :)