[sparse] allow specifying axis type in bcoo_broadcast_in_dim

[Jake Vanderplas]

Currently B[np.newaxis, :] will add a sparse axis when possible, and a batch axis otherwise.

We should add the ability to specify this. Possible API:

B[sparse.newaxis, :]  # add a sparse axis if possible, batch otherwise (same as current np.newaxis behavior)
B[sparse.newaxis.sparse, :]  # add a sparse axis, error if not possible
B[sparse.newaxis.batch, :]  # add a batch axis, error if not possible
B[..., sparse.newaxis]  # add a sparse axis if possible, dense otherwise
B[..., sparse.newaxis.dense]  # add a dense axis if possible, error if not possible

[Jake Vanderplas]

Related: #10454

[Jake Vanderplas]

I'm going to close this because I don't think we'll be adding this feature.