private List<Tensor> matrix(TensorType.Dimension.Type dimensionType, int vectorCount) {
        return matrix(3, dimensionType, vectorCount);
    }
Create a matrix of vectors (in dimension i) where each vector has the dimension x.
This matrix contains the same vectors as returned by createVectors, in a single list element for convenience.
vespajlib/src/test/java/com/yahoo/tensor/TensorTestCase.java