Add examples for Fluids models

[rafacanton]

• Improved the deep_copy method of the MeshedRegion (the generic _deep_copy in core.py paves the way for future improvements in deep_copy for other types.
• Added an example to demonstrate averaging in fluid results.

[codecov]

Codecov Report

Merging #1021 (4813dc7) into master (5b87a8a) will decrease coverage by 0.32%.
The diff coverage is 65.51%.

@@            Coverage Diff             @@
##           master    #1021      +/-   ##
==========================================
- Coverage   88.50%   88.18%   -0.32%     
==========================================
  Files          78       78              
  Lines        8915     8979      +64     
==========================================
+ Hits         7890     7918      +28     
- Misses       1025     1061      +36     

[cbellot000]

hey @rafacanton don't we want to expose realy APIs (with no strings) in the mesh info

[rafacanton]

@JennaPaikowsky This PR is ready for review. In particular, could you take a look at the examples/12-fluids files, please? Thanks!