Change the monkeypatch thing to using with contexts?

[OriolAbril]

It would be nice to be able to do things like

with matrix_dims(["dim1", "dim3"]):
    chol = xe.linalg.cholesky(da)
    eig = xe.linalg.eig(da)

instead of having to use the monkeypatch trick (currently documented) or needing to pass the dimensions every time.