Fixing post_processing plots

[germa89]

Fix the following issues:

• Empty post_processing plots Post-processing not plotting results #2288. Bug introduced in Mesh selection #2207
• Buggy colormaps plots Buggy plotting in post_processing #2296. It seems an issue with new PyVista API. It seems that passing None as cmap creates a buggy plot. We are avoiding this by setting a colormap (bwr) if no colormap is used.

Notes

PyMAPDL cmap does not work as cmap in pyvista.Plotter.add_mesh. We should fix that.

Close #2288
Close #2296

[codecov]

Codecov Report

Merging #2295 (0d8100f) into main (e3885fb) will increase coverage by 1.20%.
Report is 109 commits behind head on main.
The diff coverage is 92.45%.

@@            Coverage Diff             @@
##             main    #2295      +/-   ##
==========================================
+ Coverage   86.39%   87.59%   +1.20%     
==========================================
  Files          45       45              
  Lines        7977     8419     +442     
==========================================
+ Hits         6892     7375     +483     
+ Misses       1085     1044      -41     

[clatapie]

It correctly renders on my local installation as well!