Add support for starting values

[blegat]

Continuation for #166

This also fixes a bug: the result_index were ignored and we would invariably query the first solution.

• Test for starting values
• Test for the result_index bug
• Constrained variables

Closes #165

We were discussing this with @SophieL1. We have a use case (repo not public yet) where we need to set and get constraint attributes of a solver that is behind a dualization layer. It would be nice to have clearly defined dual_attribute, ... that allows to define how the attributes should be transformed through a dualization layer.

[codecov]

Codecov Report

❌ Patch coverage is 48.68421% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.90%. Comparing base (c347f54) to head (cce23b4).

Files with missing lines	Patch %	Lines
src/MOI_wrapper.jl	48.68%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   94.20%   89.90%   -4.31%     
==========================================
  Files          14       14              
  Lines         742      812      +70     
==========================================
+ Hits          699      730      +31     
- Misses         43       82      +39     

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[joaquimg]

needed?