Added clarification about metadata attributes in this library
Added note about intended purpose of this library
Added section on how to contribute
Added blank line for consistency
Added note about creating a virtualenv when working on this library
Use https URLs by default
Added sanity checking with regard to the puzzle dimensions
Indicate that these variables are not used
Make constants ACROSS and DOWN available to user
Fixed loading of .ipuz puzzles that do not have a solution element
Reuse iteration logic as specified by cells property
Fixed loading of .ipuz puzzles that do not have Across and/or Down clues
Only include 'block' and/or 'empty' attributes in .ipuz puzzles when they are different from the default
Only include data in .ipuz output when the values have been defined
flake8 cleanup
Expanded values in content property to ensure file-specific data is included