Add Parallel File Parsing and Improve testing

[clevelam]

This enables parallel file processing for outputs, tallies, and dumps but default.

• Add mutliprocessing tasks to parallelize multiple file parsing
• Enabled by default but can be disabled by setting an environment variable OPPPY_USE_THREADS=False
• Add log scaling option for 2d contours
• Add golds for test_dump_utiles.py, test_output.py, and test_tally.py
• Add pass/fail checking for test_interacitve
• Add log scaling testing for 2d contours
• Add 'SHOW_PLOTS' testing environment variable to hide plots while testing
• Add 'OPPPY_UPGOLDS' testing environment variable to automate the update of the new OPPPY test golds.

[clevelam]

@tylerjereddy One more request. Would you be willing to review this Change set. Primarily I would like feedback on my multiprocessing implementation. I've found that this drastically speeds up the parsing for large number file sets.

[tylerjereddy]

I added some initial comments/thoughts, a lot of it related to testing robustness to be confident in the parallel behavior.

I didn't try to check if the new functionality is actually faster, the community like using https://github.com/airspeed-velocity/asv for that, although this is quite formal so may be too much if you already have the performance envidence you need.

[clevelam]

Thanks for the feedback @tylerjereddy I've significantly updated the testing and opened a couple of follow on issues.

#35
#34