Only force skew when tensions are applied

[pmrv]

calc_md forced triclinic boxes if non-isotropic pressures were set, like (None, 0, None). This is not necessary (and inconsistent with calc_minimize), so I fixed it. I suppose the if just slipped indentations.

[coveralls]

Pull Request Test Coverage Report for Build 986437826

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.869%

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Totals
Change from base Build 983920110:	0.0%
Covered Lines:	10802
Relevant Lines:	15916

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💛 - Coveralls