Improve querying for measures

[ablaom]

There used to be measures(matching(y)) but that didn't really make sense because y doesn't tell you whether you want to running a probabilistic model or a deterministic one. @f0lie suggests basing the query on a ready-constructed machine, which makes more sense. See JuliaAI/MLJ.jl#753 (comment) .

Maybe something like measures(matching(mach))??

[ablaom]

The problem with this, of course, is that measures should ideally be conceptualised as independent of MLJ altogether, and machines are an MLJ concept.

[ablaom]

Closing as duplicate of JuliaAI/MLJ.jl#957