Default values for configuration settings

[osma]

Currently some configuration settings, especially limit, must be set, otherwise Annif will raise an exception:

  File "/home/ozone/git/Annif/annif/cli.py", line 165, in run_suggest
    hits = hit_filter(project.suggest(text, backend_params))
  File "/home/ozone/git/Annif/annif/project.py", line 175, in suggest
    hits = self._suggest_with_backend(text, backend_params)
  File "/home/ozone/git/Annif/annif/project.py", line 115, in _suggest_with_backend
    hits = self.backend.suggest(text, project=self, params=beparams)
  File "/home/ozone/git/Annif/annif/backend/backend.py", line 44, in suggest
    return self._suggest(text, project, params=beparams)
  File "/home/ozone/git/Annif/annif/backend/tfidf.py", line 55, in _suggest
    return fullresult.filter(limit=int(self.params['limit']))
  File "/usr/lib/python3.6/configparser.py", line 1233, in __getitem__
    raise KeyError(key)
KeyError: 'limit'

This is unfriendly to the user. There should be sane defaults for all settings. limit should default to 100, project name should default to its id, chunksize should default to 0 (no chunking) etc.

[juhoinkinen]

This was covered in #324 and made to release 0.43.