chore: migrate part3 to its own flat file

[dagousket]

Why?

• separate flat_main and function definition

What?

• migrate fm_generate_graph to its own flat
• update test to be save in inst/examples
• update sheme

issue #77

Checklist:

• [x] Does the package check works locally?
• [x] Does the CI pass?
• [x] Are the added / fixed features documented, tested?
• [x] Are the changes related to tickets / issues that I have listed in the commits and in the PR itself?
• [x] Are the tickets in "review" mode in the Project Tracking Board?
• [x] Does each ticket, if it is to be closed after acceptance of the PR, contain a comment that tells how to validate it?

[codecov[bot]]

Codecov Report

Merging #78 (3daeda3) into main (740eb71) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   71.92%   71.92%           
=======================================
  Files           7        7           
  Lines         716      716           
=======================================
  Hits          515      515           
  Misses        201      201           

Impacted Files	Coverage Δ
R/fm_fit_model.R	87.71% <ø> (ø)
R/fm_generate_graphs.R	41.66% <ø> (ø)

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