Add support for starting values

[blegat]

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 https://github.com/jump-dev/Dualization.jl/issues/165

[codecov[bot]]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (208b284) 94.73% compared to head (2438c31) 90.20%.

:exclamation: Current head 2438c31 differs from pull request most recent head 7bd05ee. Consider uploading reports for the commit 7bd05ee to get more accurate results

Files	Patch %	Lines
src/MOI_wrapper.jl	14.28%	36 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   94.73%   90.20%   -4.54%     
==========================================
  Files          13       13              
  Lines         703      745      +42     
==========================================
+ Hits          666      672       +6     
- Misses         37       73      +36     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.