feat: refactor domain_mesh_spe source file

[dagousket]

Why?

• call a function to create mesh

What?

• refactor script from source domain_mesh_spde into a new subfunction of fm_load_data

issue #66

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 #69 (84f74c2) into main (298c1b0) will increase coverage by 3.32%. The diff coverage is 90.32%.

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
+ Coverage   64.82%   68.15%   +3.32%     
==========================================
  Files           4        5       +1     
  Lines         543      628      +85     
==========================================
+ Hits          352      428      +76     
- Misses        191      200       +9     

Impacted Files	Coverage Δ
R/fm_build_domain_mesh_spde.R	87.67% <87.67%> (ø)
R/fm_load_data.R	87.25% <100.00%> (+1.69%)	:arrow_up:

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