reorganize discussion on parallelization and numba #367
Description
Take the first part of https://python-programming.quantecon.org/parallelization.html, up to when it starts discussing Numba, and append it to the end of need_for_speed.md.
Take the rest of that lecture and append it to numba.md.
Delete parallelization.md and remove it from the index.
Proofread and make sure everthing makes sense.
Add minimum possible changes to make the text and discussion flow.
@mmcky Would you mind to take on the first pass of this and pass it to me for review? Please feel free to delegate.