Save results in parallel for each index

[manuelma]

Use threads to save results for each variable/constraint/expression index in parallel.

Checklist before merging

• [ ] Documentation is up-to-date
• [ ] Unit tests have been added/updated accordingly
• [ ] Code has been formatted according to SpineOpt's style
• [ ] Unit tests pass

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.23%. Comparing base (b4a43d2) to head (709c602). Report is 4 commits behind head on v0.7.2.x-release.

Files with missing lines	Patch %	Lines
src/run_spineopt_basic.jl	77.77%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v0.7.2.x-release #1080 +/- ## ==================================================== + Coverage 86.21% 86.23% +0.01% ==================================================== Files 141 141 Lines 3714 3719 +5 ==================================================== + Hits 3202 3207 +5 Misses 512 512 ```

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