Gemini: get_file should be flexible about the filename

The API can return filenames from the Gemini service that end in .bz2, but the download request wants the .fits filename. This maks the intuitive use of the astroquery module fail. i.e. something like this should work as the user would expect

```
for fname in res['filename']:
Observations.get_file(fname)
```

Start at the Observations.get_file entry point and trace how Gemini filenames from res['filename'] are converted into the download request. Verify the behavior with a response whose filename ends in .bz2; done means the example loop downloads the corresponding .fits file without requiring manual filename changes.

Written by the indexing model from the issue text.