Added a KNN model and KNN model trainer

[r59q]

Adds a K-Nearest Neighbour ML model and a trainer for the model. See KNNMLModel.ts and KNNModelTrainer.ts.

 const trainModelButtonClicked = () => {
    const k = 10;
    model.train(new KNNModelTrainer(k)).then(() => {
        // Model finished training
    });
  };

<button on:click={trainModelButtonClicked}>
  train KNN model!
</button>

Theres no UI to select it yet. However one can try it out by going to the playground and use the 'train KNN model' button to train one, then return to the model tab to see how well it works. Tip: Turn off the synthesizer before.

This prepares the next task #431