Fix/plot quadratic elements

[PProfizi]

• exposes as_linear argument in plot helpers
• hide internal edges of quadratic elements -> this is actually quite difficult to achieve as there is no direct way to trigger this using PyVista hide internal edges of quadratic cells like ParaView pyvista/pyvista#867

[codecov]

Codecov Report

Merging #910 (ff4bc03) into master (3620fff) will decrease coverage by 1.74%.
The diff coverage is 88.05%.

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
- Coverage   88.82%   87.09%   -1.74%     
==========================================
  Files          73       73              
  Lines        8297     8323      +26     
==========================================
- Hits         7370     7249     -121     
- Misses        927     1074     +147     

[PProfizi]

#915 is used to merge the fixes on as_linear=True, without exposing as_linear argument in plotting methods.
The possibility to plot as_linear=False should only be merged when internal edges can be hidden, otherwise the plotting would still be wrong despite the underlying vtkUnstructuredGrid being right.