FIX: Airgap mesh creation with user defined fsl files for stator and rotor
FIX: Airgap mesh creation with user defined fsl files for stator and rotor

Actual implementation gives several errors, also it was not considered if the user has already created the airgap mesh by itself.
If so, he could now set the variable `airgap_created` to `true`.

SEMAFORInformatik/femagtools,SEMAFORInformatik/femagtools