Nearest centroid classifier

Implement:

def nearest_centroid_fit(input: np.ndarray, labels: np.ndarray) -> dict:
    """Returns {"classes": ..., "centroids": array (n_classes, n_features)}."""

def nearest_centroid_predict(model: dict, queries: np.ndarray) -> np.ndarray:
    """Returns the class of the nearest centroid to each query."""

• Reuse 01-knn's pairwise_distances, don't recompute distances by hand.