Set OMP_NUM_THREADS in preprocessing

[mattwthompson]

Might be related to #53

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[mattwthompson]

/regenerate-cache

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[mattwthompson]

/regenerate-cache

[mattwthompson]

/regenerate-cache

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[Yoshanuikabundi]

I'm in favour of this (ideally OMP_NUM_THREADS=1) iff it doesn't cause notebooks to fail. When I've experimented with setting this here in the past it's tended to cause errors. Setting it to 2 instead is something I've considered but it seemed too hacky... I like that it seems to speed things up here though. Even better would be some way to pin subprocesses of each thread in the threadpool to one core, but I can't find any way to do that.