We need some simple unit-tests here to make sure the vector is correct:
* `get_joint_feature_vector([...], [1]) == m`
* `get_joint_feature_vector([1], [...]) == n`
* `get_joint_feature_vector([...], [...]) == m*n`
* `get_joint_feature_vector([1 0 2], [0 1 0]) == [0 0 0 1 0 2 0 0 0]`
* `get_joint_feature_vector([1 2 3], [1 1 1]) == [1 2 3 1 2 3 1 2 3]`