Add semantics for specifying missing attributes are unobserved / unknown rather than known / missing.

[cmaclell]

There are at least two way to interpret missing values:

• unknown / unobserved
• known / missing

We currently assume missing values fall into the later case, but it might be nice to have the ability to say you know there is an attribute there, but it's value is unobservable, such as a masked value.

Might be nice to add the ability to handle this.