Update get_sample_names() to filter by project

[coveralls]

Pull Request Test Coverage Report for Build 5194197096

• 25 of 25 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 82.922%

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Totals
Change from base Build 5072782504:	0.3%
Covered Lines:	1600
Relevant Lines:	1811

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💛 - Coveralls

[charles-cowart]

@wasade @antgonza Just to provide some context for this change: get_sample_names() returns all of the sample-names in the user's sample-sheet or mapping-file, across all projects. I have a case where I need to compare the set of sample-names in a project with the set of sample-names in Qiita, and the addition of sample-names from other projects was corrupting the results. This fix addresses that issue.