Fix indent in out comment debug code
Add XCVersionGroup support

XCVersionGroup is used by CoreData models.

Refactor PBXGroup to have a isValidChild class method.
Implement isValidChild for PBXGroup, PBXVariantGroup and XCVersionGroup to make sure they have valid children.

Add TestCodeData target regression test to RespectTestProject.

Related and based on issue #7 reported by @cysp
Add not nil assert and error for test_PBXProjectTest

Fail fast and easy debug of errors.