Wrong key in settings for /cases/templates/flying_wings.py

Line 598 of /cases/templates/flying_wings.py currently has a wrong key of 'clean_up_previous_solution'. It should be 'cleanup_previous_solution'.

Dear Chit Yuen Lam,

many thanks for pointing this out. I have already opened a pull request to correct this typo which should be merged into the develop branch soon.

Cheers, Steffi