Fix having to plot twice to get a file with size bigger than 3kb

[germa89]

Problem was that the graphics wasn't "flushed" after executing all the commands. Now using the context manager, we make sure we flush after running each plot command.

I also fixed the static images not being plot in jupyter notebooks, the key was relaxing the regex (PNG_IS_WRITTEN_TO_FILE) which decides if the plot was written to a file or not.
Also I had to flush (mapdl.show("close")) the graphics before plotting (otherwise the 3kb file gives error).

Close #1629

[codecov]

Codecov Report

Merging #2141 (e501c06) into main (997e288) will increase coverage by 6.33%.
The diff coverage is 95.90%.

@@            Coverage Diff             @@
##             main    #2141      +/-   ##
==========================================
+ Coverage   80.82%   87.16%   +6.33%     
==========================================
  Files          44       45       +1     
  Lines        7489     8079     +590     
==========================================
+ Hits         6053     7042     +989     
+ Misses       1436     1037     -399     

[clatapie]

I think adding a test would be interesting but I let you decide @germa89 if it would be relevant or not!

[germa89]

I think adding a test would be interesting but I let you decide @germa89 if it would be relevant or not!

Unit test added in b3987bf

[clatapie]

LGTM!