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 81.01266% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.58%. Comparing base (410ae5e) to head (69c5bf2).

Files with missing lines	Patch %	Lines
src/attributes.jl	80.64%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   94.20%   91.58%   -2.62%     
==========================================
  Files          14       15       +1     
  Lines         742      820      +78     
==========================================
+ Hits          699      751      +52     
- Misses         43       69      +26     

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