BF: Fix NumPy warning when creating arrays from ragged sequences

[jhlegarreta]

Fix NumPy warning when creating arrays from ragged sequences in IVIM
reconstruction test.

Fixes

VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences
(which is a list-or-tuple of lists-or-tuples-or ndarrays with different
lengths or shapes) is deprecated. If you meant to do this, you must specify
'dtype=object' when creating the ndarray

triggered in
https://travis-ci.org/github/dipy/dipy/jobs/676101427#L5495

Rationale:
https://numpy.org/neps/nep-0034-infer-dtype-is-object.html

[skoudoro]

Thank you for this @jhlegarreta. I have one question, see below?

[codecov]

Codecov Report

Merging #2133 into master will increase coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #2133      +/-   ##
==========================================
+ Coverage   91.18%   91.20%   +0.02%     
==========================================
  Files         249      251       +2     
  Lines       32037    32214     +177     
  Branches     3386     3401      +15     
==========================================
+ Hits        29213    29382     +169     
- Misses       2085     2091       +6     
- Partials      739      741       +2     

Impacted Files	Coverage Δ
dipy/align/imwarp.py	98.39% <ø> (ø)
dipy/align/metrics.py	96.07% <ø> (ø)
dipy/align/streamlinear.py	85.25% <ø> (ø)
dipy/core/gradients.py	90.43% <90.69%> (-0.48%)	⬇️
dipy/core/tests/test_gradients.py	98.76% <97.95%> (+0.23%)	⬆️
dipy/align/tests/test_imaffine.py	99.38% <100.00%> (ø)
dipy/reconst/tests/test_ivim.py	98.02% <100.00%> (ø)
dipy/io/stateful_tractogram.py	68.85% <0.00%> (ø)
dipy/reconst/tests/test_peak_dg.py
... and 7 more

[jhlegarreta]

The failing travis build's issues are being tracked here #1821.

Otherwise, the warning this PR targets is no longer present.

[skoudoro]

Thank you @jhlegarreta! LGTM, merging