Walk_2019 has outdated version in model_files.yml

[JostMigenda]

In model_files.yml, Walk_2019 currently lists release_version: 1.2. However, those files were updated in #211 which was released as part of v1.3, so we should update the yml file.

More generally, I think this release_version property is unnecessary; we always want to use the version of the model files that matches the SNEWPY version. Instead of fixing this one entry manually, we should see whether we can avoid manually specifying the version at all.

(Thanks to @sgriswol and another IceCube collaborator for discovering this issue!)

[sgriswol]

To clarify our conversation, I only mentioned the Walk_2019 model, however every model in model_files.yml still lists release_version: 1.2. This issue was found by loading Walk_2019 and attempting to interpolate the luminosity/flux. Changes made between SNEWPY v1.3 and v1.2 (#211 ) fixed this issue.

An implication of this current issue is that, if model_files.yaml isn't updated appropriately when changes are made to any model, those changes will be missed when the user attempts to download the models.

[JostMigenda]

Quick note that this also affects Zha_2021 now, see #239.

[JostMigenda]

Note that this might be resolved as part of #250.