Allow option to turn off numba parallelization

[alex-l-kong]

Describe the bug

On some devices, running relabel_segmentation with nb.njit(parallel=True) will crash the kernel. Setting parallel=False should fix this issue. There should be an option to allow users to turn off parallelization without having to manually copy over code.

Expected behavior

Reliability should be enforced and the code should run, even if a suboptimal routine needs to be used.

To Reproduce

It's most frequently seen on a 16 GB RAM computer. I think it's best to limit the number of times we tell users "get a bigger computer", as 16 GB is small but not unheard of.