added a fat lock around numexpr.evaluate

[jennolsen84]

fixes #80

[FrancescAlted]

Hmm, with this fat lock maybe there is no need for #199 . Can you confirm please? It would be a good idea to put the lock as high as possible and then remove the ones that are not needed.

[jennolsen84]

Perhaps, but the extra locks aren't that bad in this case. Lock contention hurts, but here we'll be avoiding lock contention. Locks also prevent some minor optimizations from happening by the C compiler (e.g. caching a variable in a register) across memory barriers (which a lock will insert), but that hit is so minor at this point, it is not worth mentioning.

Please accept this PR, as things are better with it than without.

[FrancescAlted]

Ok. So merging.