Unique names don't require underscores

[matbesancon]

All models end up marking each variable group with _$i, even though the variables themselves are often in containers.

This typically ends up with a double indexing scheme in JuMP:

: π[6]_1 ≥ -8006.0
 : π[5]_1 ≥ -8005.0

This PR adapts it to avoid the _i if the variable name is unique

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (14edeae) 94.73% compared to head (a00761a) 94.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
+ Coverage   94.73%   94.76%   +0.02%     
==========================================
  Files          13       13              
  Lines         703      707       +4     
==========================================
+ Hits          666      670       +4     
  Misses         37       37              

Impacted Files	Coverage Δ
src/dual_model_variables.jl	93.39% <100.00%> (+0.25%)	:arrow_up:

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