JOSS Review: simplify and update figure 2 in paper

[kmongird]

Purpose:

JOSS Review:
Additional comment added by reviewer: "The diagram shown Fig. 2 is quite complicated and it is not easy to relate it to the text you added in this section. I think it might be simplified to better show the link with the text (you have numbers in the text which can not be found in the figure for example)"

Related:

Review openjournals/joss-reviews#4336

Resolved:

Resolve #8

[codecov-commenter]

Codecov Report

Merging #15 (a257df8) into main (84be817) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   87.05%   87.05%           
=======================================
  Files          15       15           
  Lines        3392     3392           
=======================================
  Hits         2953     2953           
  Misses        439      439           

Flag	Coverage Δ
unittests	87.05% <ø> (ø)

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