[DOC]: Fix inconsistent function parameters

[areeshatariq]

Some parameters were inconsistent e.g. static_img_files and static_image_files. They are made consistent to avoid any confusion.

[jhlegarreta]

Looks good to me. Thanks for doing this.

[codecov]

Codecov Report

Merging #2295 (65ed2ec) into master (6c54e89) will decrease coverage by 1.99%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2295      +/-   ##
==========================================
- Coverage   91.38%   89.39%   -2.00%     
==========================================
  Files         254      254              
  Lines       33850    33850              
  Branches     3569     3569              
==========================================
- Hits        30933    30259     -674     
- Misses       2111     2815     +704     
+ Partials      806      776      -30     

Impacted Files	Coverage Δ
dipy/workflows/align.py	88.12% <100.00%> (ø)
dipy/workflows/tests/test_viz.py	23.80% <0.00%> (-73.02%)	⬇️
dipy/viz/app.py	4.40% <0.00%> (-72.87%)	⬇️
dipy/viz/tests/test_apps.py	24.56% <0.00%> (-70.18%)	⬇️
dipy/viz/panel.py	3.41% <0.00%> (-60.76%)	⬇️
dipy/io/vtk.py	28.57% <0.00%> (-60.72%)	⬇️
dipy/workflows/viz.py	14.28% <0.00%> (-48.58%)	⬇️
dipy/viz/__init__.py	35.00% <0.00%> (-30.00%)	⬇️
dipy/io/tests/test_streamline.py	89.65% <0.00%> (-8.05%)	⬇️
dipy/utils/optpkg.py	73.91% <0.00%> (-4.35%)	⬇️
... and 6 more

[jhlegarreta]

The failure reported by Travis CI is unrelated. Merging.