Add post execution hook with preview

[brandomr]

Simple PR to add the post execution hook with the model response (not the full configuration wrapper) in the mira_config_edit context.

This also ensure support for previews of regnet + stock and flow on both the mira_config_edit context as well as the mira_model context.

[Tom-Szendrey]

LGTM!
Thank you!

Here I have ran an update to set beta to 0 and gamma to 1 and got the expected response:
Sample updated model config.json

[brandomr]

@Tom-Szendrey I went ahead and updated the model_preview.py in both mira_model and mira_config_edit contexts to ensure appropriate handling for regnet + stock and flow. im not sure whether the image will be generated correctly though so am testing now for regnets.

[Tom-Szendrey]

Thanks Brandon! Can we merge this?