use lsst function to disable implicit threading

[jchiang87]

No description provided.

[rmjarvis]

A quick inline comment about why this is useful is probably appropriate here.
Something like:

# We never use threading in imsim, and without this, some systems try to run all the 
# multiprocessing jobs on a single core, which leads to terrible performance.
# We think disabling this is never a problem, and sometimes extremely important.
# It's best if this is done as soon as possible, hence here before any other imports.