feat: refactor model fitting as a function

[dagousket]

Issues to validate to close :

• [ ] issue #27

Checklist:

• [x] Does the package check works locally?
• [x] Does the CI pass on Ubuntu?
• [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 #29 (ef7deb4) into main (c7b728a) will decrease coverage by 2.41%. The diff coverage is 79.71%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
- Coverage   84.52%   82.11%   -2.41%     
==========================================
  Files           1        2       +1     
  Lines          84      151      +67     
==========================================
+ Hits           71      124      +53     
- Misses         13       27      +14     

Impacted Files	Coverage Δ
R/fm_fit_model.R	78.78% <78.78%> (ø)
R/fm_load_data.R	84.70% <100.00%> (+0.18%)	:arrow_up:

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