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.
Merge pull request #2115 from pllim/vo-cat-io

Expand functionalities for VO catalog and database classes