Prevent import of ctapipe.image at import time of ctapipe.io

[maxnoe]

Import ctapipe.image.toymodel in ctapipe.io.toymodel only when used, not directly at import time to improve import time of ctapipe.io when not using the toymodel.

[codecov[bot]]

Codecov Report

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

Comparison is base (6ffc12d) 92.45% compared to head (bb8d0ee) 92.47%. Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2488 +/- ## ========================================== + Coverage 92.45% 92.47% +0.01% ========================================== Files 234 234 Lines 19956 19965 +9 ========================================== + Hits 18451 18462 +11 + Misses 1505 1503 -2 ```

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