Abstract code that builds file paths from urls into a function, using os.path. and urlsplit

This would be used in the process_go.py, process_kegg.py and process_do.py files.

As per Matt's suggestion, it could look something like this:
```
def filename_from_url(self, folder, urlstr):
return os.path.join(folder, 'DO', os.path.basename(urlsplit(urlstr).path))
```

This issue was imported from the old repository in bitbucket (https://bitbucket.org/greenelab/annotation-refinery/issues).

Start by reading the path-building code in process_go.py, process_kegg.py, and process_do.py, then compare the repeated URL handling with the proposed os.path and urlsplit helper. Done means the shared function is used by all three files while preserving their intended output paths.

Written by the indexing model from the issue text.