Please provide support for sort in 2-D field!

[mushroomfire]

I think a built-in function for sorting a 2-D field will be useful for taichi user, just like np.argsort in numpy, that will be great! Now ti._kernels.parallel_sort seems to only be effective for 1-D field.

[classAndrew]

Hi, I'd like to implement this feature as a first contribution.

[SigmaAdrich]

Hi, I'd like to implement this feature as a first contribution. If you implement this feature, please let me know as soon as possible. Thank you very much.