setNumThreads() as context manager

[Matthew Spellings]

It might be kind of nice to be able to set the number of threads freud will use in parallel bits through a context manager.

Example:

rdf = freud.RDF(...)
# uses all threads
rdf.compute(...)

with freud.parallel.numThreads(1), multiprocessing.Pool() as p:
    p.map(analysis_function, frames_to_analyze)

[Eric Harper]

Resolved