Add parallel processing

[mikkokotila]

You want to make a PR to Wrangle

Thanks so much :) First, please take a moment to carefully check through
the below items:

Sanity

• I'm aware of the implications of the proposed changes
• Code is PEP8
• I'm making the PR to master

Docs

• Docs are updated
• Docs version is correct (index.html and _coverpage.md)

Tests

• Changes have gone through actual use testing
• All local tests have passed (run ./test.sh in /talos)
• Tests have been updated to reflect the changes

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[pep8speaks]

Hello @mikkokotila! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file wrangle/df/df_parallelize_process.py:

Line 2:1: W293 blank line contains whitespace
Line 4:1: W293 blank line contains whitespace
Line 9:1: W293 blank line contains whitespace
Line 13:1: W293 blank line contains whitespace
Line 19:1: W293 blank line contains whitespace

[coveralls]

Pull Request Test Coverage Report for Build 112

• 2 of 11 (18.18%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 50.874%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wrangle/df/df_parallelize_process.py	1	10	10.0%

Totals
Change from base Build 107:	-0.3%
Covered Lines:	611
Relevant Lines:	1201

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💛 - Coveralls