Document GetFermilatDatafile

TODO in astroquery/astroquery/fermi/core.py (121)
```
class GetFermilatDatafile(object):
"""
TODO: document
TODO: Fail with useful failure messages on genuine failures
(this doesn't need to be implemented as a class)
"""
```

Open astroquery/astroquery/fermi/core.py at line 121 and read GetFermilatDatafile together with its surrounding code and usages. Document the class according to its current behavior, remove the documentation TODO, and verify that the resulting docstring accurately describes what the class does; the failure-message TODO is a separate concern.

Written by the indexing model from the issue text.