Adding MulticlassPerceptron model

[davidbp]

Hello,

I registered a perceptron implementation with an interface to MLJ. I would like to add it to MLJ. I am sure there will be details to improve but so far I have my tests running with the current MLJ version.

[ablaom]

I remember some back and forth on this, but without follow through on the part of the model provider. So closing as stale.

BTW: MLJFlux models presumably provide the same functionality, and are now quite mature.