Description

Since ScalarValue objects now have base attributes (as they can have
descriptions, preferredName and see references), they should also correctly
return their source file when getSourceFile() is called.

Fixes #726

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• [-] I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works