Observation dependent variables interfaces

[SamFayolle]

The dependent variable filtering option is now implemented. @DominicDirkx I have not created a similar dependent variable-based observation set splitter, because I could not really identify an immediate use case for this. But just let me know if you think otherwise :)

[DominicDirkx]

Cool, thanks! I don't really see (for now) much of a use of a set-splitter, let's leave it for now.

I see you commented out some of the test cases in unitTestObservationsProcessing.cpp, I assume to speed up your local compilation and run (?) Except for adding this back, is there anything that you want to add before we can merge this?

[SamFayolle]

I see you commented out some of the test cases in unitTestObservationsProcessing.cpp

Yes, I realised that too, sorry! They will be back with the next commit

Except for adding this back, is there anything that you want to add before we can merge this?

nothing feature-wise, but I thought I'd spend some time adding some proper comments this afternoon, because that's really missing

[SamFayolle]

For now, I only added comments to the new dependent variables stuff but I realised that the whole observations.h file could use a lot more comments in general (also for the new parser/filter/splitter functionalities). I don't think that it is necessary for this PR though(?) but something to keep in mind

[DominicDirkx]

Merged :)