Prevent VOTable validator from crashing when system sends SIGKILL during xmllint.
Fix #2398.  Support a mix of strings and quantities passed to the angle constructor.
Expanded VO catalog and database functionalities. Moved VO exceptions to a separate modules in client and validator. Changed inheritance to super. Updated doc and tests. Fixed codes for Python 3 compatibility and to complete coverage. Applied review comments.
Reduced default search radius for cone search validation to avoid large query results for xmllint.
Fixed failing tests because they do not always run in order.
Follow on to #2405 that handles some Python3-specific issues that slipped through the cracks
Merge pull request #2398 from mdboom/coordinates/mixed-strings

Different input formats for Angle confuse unit parsing
Merge pull request #2115 from pllim/vo-cat-io

Expand functionalities for VO catalog and database classes
Don't generate the config file any longer, just keep it in the repo and
copy it over.
Add a test.
Remove save functionality from config
First pass at moving around all of the configuration options.  Tests passing.
Don't warn on import
Notate the version at which configuration items were renamed.
Fix more warnings on import.
Minor docstring improvements.
Make configuration items show up in docs
Update config doc pages.
Document configuration changes.
Reorganize astropy.cfg based on general categories in docs.