Issue #966: PatientData interface should have a get method that is not integer based.

I completely agree that making `PatientData` extend `List<ImmutablePair<String, T>>` is not at all the right way to go.
`PatientData` does need a getter method. The best other solution I've come up so far is `PatientData<T,  SL, RT> ` where `T` is the type of data the list holds, `SL` is the type by which search is performed and `DT` is the type that is returned when searched.

A list of measurements without any names anywhere makes no sense to me. How do you know which measurement is which?