You know, I think it might be worth printing a deprecation warning here before the `subprocess.call()`, and do the same below in galsim_yaml... Some of the usage message that gets printed if you just run `galsim_json` or `galsim_yaml`, specifically
```
Works with both YAML and JSON markup formats.
```
seems a little incongruous, as does the ambiguity over what might happen if you followed the usage message and tried `galsim_yaml -f json` for example.  A deprecation warning would be a fair enough alternative to us having to cover any corner cases.  What do you guys think?