Description of proposed changes

• image_stitch: update REF_Y/X after stitching, and/or remove SUBSET_Y/XMIN/MAX

• save_kmz: use a different data png file name to avoid conflict with / overwrite view.py's default output file name

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.