grid_helper_curvelinear cleanups.

[anntzer]

• np.full already defaults to the dtype of the value passed in, no
  need to explicitly pass it.
• set_extremes already normalizes None to +/-inf, no need to handle
  them again in get_tick_iterators. Also, set_extremes and
  update_lim already clearly assume that extremes are sorted, so
  unsorted extremes already break elsewhere, so no need to sort them in
  get_tick_iterators.
• There's a public setter for extremes, no need to touch private
  attributes in test_axis_direction.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).