Segmentation Fault with VECs when multithreaded

[sestephens73]

In the QR-factorization example, we spawn multiple threads for calling into VECs. Each VEC then calls into its own NumPy thread pool to do a qr factorization on its block. A segmentation fault occurs during this process. It's more likely to happen when more threads calling VECs are used.

https://github.com/ut-parla/Parla.py/blob/991f03698364c477ca04de0819cbafb7b8e481b1/examples/qr_factorization/vec-segfault.py#L45-L63