Fix all F401 unused imports

[rht]

Detected via flake8 quantecon | grep F401 | grev -v __init__.

[coveralls]

Coverage increased (+0.01%) to 93.817% when pulling 5524a47 on rht:F401 into 6c40552 on QuantEcon:master.

[rht]

I have E501 and E231 cleanup incoming (they are automated via autopep8, so can be quickly redone, unlike F* errors), but will push them in batches once this is merged.

[mmcky]

thanks @rht -- sorry I have been away for a week. I'll have a close look tomorrow and merge.