add diagnostic plots

[lea-hagen]

This adds code to create some diagnostic plots:

• Completeness vs A_V for a subset of the stars (because all of the stars would be too many points)
• histograms of A_V for each of the spatially reorganized pixels
• also an option to make A_V histograms using chi^2 cut(s)

This is added to the example wrapper, and I put a few lines about it in the documentation.

[coveralls]

Pull Request Test Coverage Report for Build 27

• 5 of 82 (6.1%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.7%) to 18.75%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
megabeast/plot_input_data.py	5	82	6.1%
<!--	Total:	5	82	6.1%	-->

Totals
Change from base Build 26:	-4.7%
Covered Lines:	57
Relevant Lines:	304

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💛 - Coveralls

[lea-hagen]

@karllark This is ready to merge (assuming no changes should be made)

[karllark]

Great. Merging.