@property<EOL><INDENT>def required_metadata(self):<DEDENT>
return []<EOL>
Names of metadata fields that must be present for a valid project.

Make a base project as unconstrained as possible by requiring no
specific metadata attributes. It's likely that some common-sense
requirements may arise in domain-specific client applications, in
which case this can be redefined in a subclass.

:return Iterable[str]: names of metadata fields required by a project