Still -1 values for 'unavailable' meta features

[janvanrijn]

These should be null values

[joaquinvanschoren]

Wasn't this fixed?

[janvanrijn]

I think the only way to figure this out is to actually iterate over all data qualities in code and report on that. I believe I fixed it for some, but never have enough time

[janvanrijn]

I think the last of these will be fixed once PR #14 is merged