Do not include negative values in the Gaussian prior

[sblunt]

This was causing unphysical (negative) stellar masses to be returned. Fixed.

[coveralls]

Pull Request Test Coverage Report for Build 622

• 11 of 11 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 85.279%

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Totals
Change from base Build 604:	0.2%
Covered Lines:	672
Relevant Lines:	788

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💛 - Coveralls

[sblunt]

Ok, just added that keyword. Lmk what you think now!