feat: allow setting solver backend early

[Midnighter]

Setting the solver backend before creating a model saves time (the model doesn't have to be recreated), and has other advantages, too.

• [x] description of feature/fix
• [x] tests added/passed
• [x] add an entry to the next release

[codecov[bot]]

Codecov Report

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

Comparison is base (6fc7a44) 74.86% compared to head (d0934d9) 74.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #759 +/- ## =========================================== + Coverage 74.86% 74.94% +0.08% =========================================== Files 50 50 Lines 2956 2962 +6 Branches 669 669 =========================================== + Hits 2213 2220 +7 + Misses 649 648 -1 Partials 94 94 ```

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