NF: Check multi b-value

[arokem]

Some models require that you have more than one (non-zero) b-value in your gradient table.

This new feature implements a simple function that checks a gradient table, to see that it has enough different b-values.

[codecov-io]

Codecov Report

Merging #1221 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   85.91%   85.92%   +0.01%     
==========================================
  Files         221      221              
  Lines       27247    27270      +23     
  Branches     2782     2785       +3     
==========================================
+ Hits        23408    23431      +23     
  Misses       3154     3154              
  Partials      685      685

Impacted Files	Coverage Δ
dipy/core/gradients.py	97.32% <100%> (+0.32%)	⬆️
dipy/core/tests/test_gradients.py	98.12% <100%> (+0.13%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0297078...d9fbe30. Read the comment docs.

[coveralls]

Coverage increased (+0.01%) to 88.424% when pulling d9fbe30 on arokem:check-multi-b into 0297078 on nipy:master.

[RafaelNH]

This looks good! I will merge this - it is only one added function and it is already well tested! I will add this into dki and fwdti!