v0.3.1

• Query methods are no longer static methods to make them consistent with
  other operations.
• Retrieve methods added that allow for a record to automatically
  be saved as a file after it was queried.
• The list of most stable isotopes added to atomic info to avoid having to
  specify isotope numbers for radioactive elements.
• Improvements to cache handling for local databases.
• The order of parameters for database methods were rearranged for consistency
  and practicality.
• All default query parameter values are now None to avoid unwanted automatic
  filtering of records.
• Additional bug fixes identified during testing