adds B/2 term to Paasch element, fixes #251

[mdmurbach]

Looks like we're missing the final term in equation 22 from the Paasch paper. This adds in B/2 to both the element code, docstring, and tests.

[coveralls]

Pull Request Test Coverage Report for Build 4226976993

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

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Totals
Change from base Build 4226528233:	0.0%
Covered Lines:	848
Relevant Lines:	848

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💛 - Coveralls

[BGerwe]

Thanks for fixing these. I'll have time to review this and #255 in the next day or two.