bugfix: fixes potential nonmatching b0mask with bvalues above 0.

[rutgerfick]

Closes #106

[coveralls]

Pull Request Test Coverage Report for Build 725

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 84.105%

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Files with Coverage Reduction	New Missed Lines	%
dmipy/distributions/tests/test_bingham.py	2	89.04%
<!--	Total:	2		-->

Totals
Change from base Build 707:	0.03%
Covered Lines:	5054
Relevant Lines:	5871

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💛 - Coveralls