* add check for missing input ArrayArgs and test

* fix tests that fail because of gh-450 (is_input issue)

* appease flake8

* Specify is_input in tutorial where necessary

* rename function, guard __debug__

* add arg check to CKernelExecutor

* add comment explaining placement of missing array arg check

* Improve missing-arg error message

Co-authored-by: Andreas Kloeckner <inform@tiker.net>