Ensembles: don't expect `OptimizerResult.id` to be convertible to `int`

[dweindl]

Fixes Ensemble.from_optimization_{history,endpoints}, which incorrectly assumed that OptimizerResult.id is always convertible to int.

Closes #1349.

[codecov-commenter]

Codecov Report

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

Project coverage is 83.03%. Comparing base (ace9b8b) to head (e6e45d3).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1351 +/- ## =========================================== - Coverage 84.22% 83.03% -1.20% =========================================== Files 157 157 Lines 12870 12870 =========================================== - Hits 10840 10686 -154 - Misses 2030 2184 +154 ```

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