Get single or set of row's activity

[TendTo]

Major

• Added two commodity overloads on the method computePrimalActivity to selectively compute only the activity of a single row or selected rows.
• Added getRowPrimalValue along with several variations to the solver. The idea is to have a way to efficiently get the value of a row computed with the current primal solution.

Minor

• Use default destructor definition in SPxException to avoid the warning: warning: definition of implicit copy constructor for 'SPxException' is deprecated because it has a user-declared destructor [-Wdeprecated]
• Use const reference instead of const copy for the what() getter in SPxException

[leoneifler]

Hi, thank you for submitting this PR. I will add some comments to the code.

Just so you know our current workflow: Once we think the code is good, I will cherry-pick your PR onto our internal GitLab, as that is where all of our CI runs (migrating fully to GitHub is planned, but the CI migration proves difficult). I will then close your PR, and the changes will be mirrored to GitHub (your commits will still be in your name, of course)

[leoneifler]

Could you add a CHANGELOG entry? Otherwise the PR looks good to me.

[leoneifler]

I will close this MR and move the commits over to our internal gitlab (our CI mostly runs there still so that makes this ugly workflow necessary). I will ping you once it appears here on GitHub.