Refactor get_reason

[kellertuer]

This resolves #389. I had a bit of a boring train ride today and worked through all stopping criteria, removed .reason fields and when necessary introduced values to store obtained values being printed when generating the string.

There are two small things left

• [x] for the Lagrange Multiplier criterion in the bundle methods the symbols should maybe be strings since they might also be -\xi (and not just \xi like now)
• [x] somehow I have to check what happens with JuMP their 3 checks for reasons return now empty strings and I do no directly see why

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.77%. Comparing base (4ec57f5) to head (ce56f47).

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```diff @@ Coverage Diff @@ ## master #395 +/- ## ======================================= Coverage 99.77% 99.77% ======================================= Files 72 72 Lines 7491 7586 +95 ======================================= + Hits 7474 7569 +95 Misses 17 17 ```

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[mateuszbaran]

• for the Lagrange Multiplier criterion in the bundle methods the symbols should maybe be strings since they might also be -\xi (and not just \xi like now)

That can be saved in a separate field in the criterion but leaving the string in this one place would be fine too.

[kellertuer]

Oh, I missed one solver, in CMA-ES, which has quite a few stopping criteria, they always returned a reason even if they were not active.

For now I think I did rewrite them, but testing them is still missing. We might take the chance and change the is_active fields in these and change them to at_iteration in order to provide the iteration number in the reason (at which the stopping criterion indicated to stop).

[mateuszbaran]

We might take the chance and change the is_active fields in these and change them to at_iteration in order to provide the iteration number in the reason (at which the stopping criterion indicated to stop).

Yes, I think changing it to at_iteration would make sense.

[kellertuer]

We might take the chance and change the is_active fields in these and change them to at_iteration in order to provide the iteration number in the reason (at which the stopping criterion indicated to stop).

Yes, I think changing it to at_iteration would make sense.

Did that. Also tried to use the same tests for get_reason as for others and noticed that on construction, the StopWhenEvolutionStagnatess show method errors (not affecting tests for now, but on REPL it did); since one of the means to compute is (on construction) over an empty array.

[kellertuer]

Hopefully also fixed the last error I mentioned above, which was mainly that the median of an empty vector does not exist, so I introduced a check and tested that as well.