Make gb.ss.concat work with vectors!

[eriknw]

When creating a Matrix, a Vector is handled as an Nx1 Matrix.

I still need to update the docstring for usage.
I would also like to add v.ss.split for Vectors.

For example:

>>> gb.ss.concat([v, v])  # Vector, size=2*N
>>> gb.ss.concat([[v], [v]])  # Matrix, nrows=2*N, ncols=1
>>> gb.ss.concat([[v, v]])  # Matrix, nrows=N, ncols=2

[eriknw]

Now that Matrix objects have ._parent attribute that are responsible for freeing memory, we could make from_pygraphblas not invalidate the pygraphblas Matrix. They could both remain valid while using the same underlying GraphBLAS object.