add warning and coerce if nx graph passed to solver

[tlambert03]

closes #67

[codecov[bot]]

Codecov Report

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

Project coverage is 93.35%. Comparing base (6a9e6d5) to head (742017f). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #80 +/- ## ========================================== + Coverage 93.29% 93.35% +0.05% ========================================== Files 32 32 Lines 776 783 +7 ========================================== + Hits 724 731 +7 Misses 52 52 ```

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