BF: Fix NumPy warning when creating arrays from ragged sequences

[jhlegarreta]

Fix NumPy warning when creating arrays from ragged sequences in streamline 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
orig_streamlines_world = list(np.subtract(new_streamlines_world, disps))

triggered in
https://travis-ci.org/github/dipy/dipy/jobs/676101427#L5569
and
https://travis-ci.org/github/dipy/dipy/jobs/676764173#L5561

for example.

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

[jhlegarreta]

I left this behind in PR #2133. I am sorry. I think this PR fixes the remaining warnings of that kind.

[codecov]

Codecov Report

Merging #2140 into master will increase coverage by 0.00%.
The diff coverage is 95.31%.

@@           Coverage Diff           @@
##           master    #2140   +/-   ##
=======================================
  Coverage   91.20%   91.21%           
=======================================
  Files         251      251           
  Lines       32214    32229   +15     
  Branches     3401     3403    +2     
=======================================
+ Hits        29382    29397   +15     
  Misses       2091     2091           
  Partials      741      741           

Impacted Files	Coverage Δ
dipy/align/imwarp.py	98.39% <ø> (ø)
dipy/align/scalespace.py	92.64% <ø> (ø)
dipy/io/stateful_tractogram.py	68.85% <50.00%> (ø)
dipy/core/gradients.py	90.47% <88.88%> (+0.04%)	⬆️
dipy/nn/model.py	92.59% <93.75%> (ø)
dipy/nn/tests/test_tf.py	88.46% <93.75%> (ø)
dipy/core/tests/test_gradients.py	98.76% <100.00%> (ø)
dipy/direction/peaks.py	83.41% <100.00%> (ø)
...ection/tests/test_closest_peak_direction_getter.py	100.00% <100.00%> (ø)
dipy/io/tests/test_stateful_tractogram.py	92.42% <100.00%> (ø)
... and 15 more

[skoudoro]

LGTM, Thank you @jhlegarreta, merging!