Consider renaming Assertion

[rofinn]

#69 Introduced an abstract Assertion type to define a set tests/checks/assumptions about datasets containing missings. Perhaps there's a more appropriate name than just "assertion" for that?

[rofinn]

I suppose this could also be called a MissingDataCheck and you could call check(...) (vs assert(...))? Alternative, I could see something like Impute.Validator which implements isvalid(data, ::Validator) -> Bool and verify(data, ::Validator) -> data which might remain relatively general, but doesn't overlap with @assert in base? I do feel like these should be used like traditional assert statements on assumption about the input and outputs to a functions though?