[Add] metadata-name accessor, [Resolve] issue #1592

[g5t]

Resolves #1592 by changing the return value of metadata_table_defined to 0 if the provided key is empty and adding documentation for the metadata accessor methods.

Adds a method that returns the first-defined metadata name for a known source, e.g., the component instance name. This is mainly a convenience function for users of a new component that can, e.g., make an instr file using OFF geometry component(s) self-contained.