New syntax for variable and factors

[stecrotti]

The syntax is changed in order to free the factor(s) and variable(s) keywords to be used as Julia variable names.
variable -> v_vertex
factor -> f_vertex
variables -> eachvariable
factors -> eachfactor

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.74%. Comparing base (eb5cf4e) to head (0d29199).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   84.61%   84.74%   +0.13%     
==========================================
  Files           5        5              
  Lines         117      118       +1     
==========================================
+ Hits           99      100       +1     
  Misses         18       18              

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