Fix FieldsContainer.animate for specific cases.

[PProfizi]

Fix the animation of a multi-component field without a "deform_by" argument.
Raise an error if fields_container obtained from the evaluation of a result or operator "deform_by" argument is not of the same length as the FieldsContainer being animated.
Improve example.

[codecov]

Codecov Report

Merging #570 (4033543) into master (46c9892) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #570      +/-   ##
==========================================
+ Coverage   86.84%   86.98%   +0.13%     
==========================================
  Files          67       67              
  Lines        7221     7220       -1     
==========================================
+ Hits         6271     6280       +9     
+ Misses        950      940      -10     

[cbellot000]

If I understood correctly the test_animator have no assert ? They only check that it didn't throw?

[PProfizi]

If I understood correctly the test_animator have no assert ? They only check that it didn't throw?

Some basic ones do when checking simple things, but otherwise no, as animating a FieldsContainer does not return anything, except for a potential GIF.