Improve ``DataObject`` print

[GuillemBarroso]

Fix #235.

Update FieldsContainer description to match the new post guidelines:

• fields are now displacements, stresses, velocities, pressure, etc
• FieldsContainer are now the DataObject

[codecov[bot]]

Codecov Report

Merging #238 (0e9dd0e) into post_refactor (89fdadf) will increase coverage by 3.40%. The diff coverage is 79.59%.

@@                Coverage Diff                @@
##           post_refactor     #238      +/-   ##
=================================================
+ Coverage          74.89%   78.29%   +3.40%     
=================================================
  Files                 30       30              
  Lines               1665     1714      +49     
=================================================
+ Hits                1247     1342      +95     
+ Misses               418      372      -46     

[PProfizi]

Closing as stale. Working on the specs for prints.