`image_stitch`: update `REF_Y/X` after mosaicking in the output file

[yunjunz]

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

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