Revert "feat(wof_extract) Remove dependency on gnu parallel"

[Joxit]

Reverts #134

It seems that this PR introduced a bug in wof_extract. xargs outputs are interleaved when we use it in parallel (see #142).
xargs can't solve this problem alone, there are some post on this here and here.
parallel seems to be the only viable alternative in a multi-process context.

Fixes #142

[missinglink]

Cool, thanks for the quick turnaround, it's a pity that xargs couldn't do it, I'd prefer to not have to introduce more dependencies but it is what it is :)