Resolved #66 by added KDE objects to output

[prmiles]

• User can output kde objects by specify kwarg return_kde=True when using plot_density_panel method.

[coveralls]

Pull Request Test Coverage Report for Build 415

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 97.569%

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Totals
Change from base Build 411:	0.004%
Covered Lines:	2328
Relevant Lines:	2386

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💛 - Coveralls