volume slider fix

[maharshi-gor]

This PR enables the volume slider to be functional. For nii data with volume dimension.

[codecov]

Codecov Report

Merging #2966 (c8c8b18) into master (7850354) will decrease coverage by 0.03%.
Report is 187 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2966      +/-   ##
==========================================
- Coverage   81.72%   81.69%   -0.03%     
==========================================
  Files         147      147              
  Lines       20480    20484       +4     
  Branches     3276     3277       +1     
==========================================
- Hits        16737    16735       -2     
- Misses       2912     2916       +4     
- Partials      831      833       +2     

Files	Coverage Δ
dipy/viz/horizon/tab/slice.py	52.07% <0.00%> (ø)

... and 3 files with indirect coverage changes