Remove dependence on psutil. Add utility functions for getting system memory.

[robertnishihara]

This removes the dependence on psutil. We really only needed psutil to get the cpu count, which we can do with multiprocessing, and to get the memory size. Memory size is a bit trickier. I used some code from the psutil tests to try to do the same thing.

This fixes #2806.

To look into before merging:

• Try on more platforms (e.g., CentOS).
• Make sure license is ok (psutil is BSD I believe)

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[raulchen]

LGTM. Please let me know when the todos are done, then I'll merge it.

[robertnishihara]

@raulchen Thanks, I think this is ok to merge.