[simulator] Look up decoding simulation works on linux distro but not on OSx

[Phionx]

The simulation tutorials/rep/2-fitters-lookup.ipynb requires errors being inserted between the stabilizer measurements to produce a lookup table of possible results. This works on elementary OS, but not on OSx.

Expected Result (elementary OS):

Incorrect Result (Mac OSx):

This is probably an issue with the aer_simulator.

[Phionx]

It turns out this is because the simulator is optimizing away the identity gates we place where we wish to insert single errors. So, as a quick fix, we can set the optimization_level=0 in execute to prevent this optimization. This may not be the most efficient solution, and we can continue to work on speeding up the simulator in #64 .