Speed up the execution time

[heronyang]

Problem

Currently, a single simulation on 9 hours scenario data, for sfo-terminal-2, and rescheduling on each tick takes around 2 to 3 minutes. It's too slow for us to run lots of batch tests.

Solution

1. Use the profiling technique described in the README to find out the slow parts of the code, and optimize it.
2. Use multiple thread technique for the batch runs since there's no shared data between two simulation runs. (The surface/scenario data may be shared, but it's not hard to avoid that).
3. Replace the slow code with c++ or c code. Or, pypy.

Note

pypy doesn't support numpy/pandas plots at this point. We will need to figure a way to avoid this if we're optimizing the code with pypy.