fixes gtab dmipy2dipy bug with undefined deltas

[rutgerfick]

Closes #78

[coveralls]

Pull Request Test Coverage Report for Build 588

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 82.527%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dmipy/core/acquisition_scheme.py	4	6	66.67%
<!--	Total:	4	6	66.67%	-->

Files with Coverage Reduction	New Missed Lines	%
dmipy/core/acquisition_scheme.py	1	81.43%
<!--	Total:	1		-->

Totals
Change from base Build 586:	-0.07%
Covered Lines:	9399
Relevant Lines:	11086

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #81 into master will decrease coverage by <.01%. The diff coverage is 16.66%.

@@            Coverage Diff            @@
##           master     #81      +/-   ##
=========================================
- Coverage   81.71%   81.7%   -0.01%     
=========================================
  Files          64      64              
  Lines        5539    5543       +4     
  Branches      653     655       +2     
=========================================
+ Hits         4526    4529       +3     
  Misses        840     840              
- Partials      173     174       +1

Impacted Files	Coverage Δ
dmipy/core/acquisition_scheme.py	78.43% <16.66%> (-0.76%)	:arrow_down:
dmipy/distributions/tests/test_bingham.py	98.5% <0%> (+4.47%)	:arrow_up:

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