Minor updates to PR

[kbonney]

Summary

• Removed mutable default argument in _extract_dataframe.
• Fixed a few places in the docs where truncate was used instead of truncate_names for the valid_gis_names argument.
• Clarified the use of complete_list in the user guide and docstring for valid_gis_names.

Tests and documentation

Acknowledgement

By contributing to this software project, I acknowledge that I have reviewed the software quality assurance guidelines and that my contributions are submitted under the Revised BSD License.

[kbonney]

Not sure why tests are failing. I am seeing the failures ocurring in test_graphics, which shouldn't have anything to do with these changes.