improve the type&conversion handling around IO

[exaexa]

should close #62

[codecov-commenter]

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Codecov Report

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

Project coverage is 100.00%. Comparing base (6b261e9) to head (fc313b7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #63 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 37 37 Lines 544 548 +4 ========================================= + Hits 544 548 +4 ```

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[exaexa]

@stelmo does the save_converted_model name sound OK? maybe it should say save_model_converted_by_path_extension but man, naming is hard

:de: variant: speichern_mit_dateinamenerweiterungkonvertierung, wonderful how this can be managed in 3 simple words