Returns (at runtime) the absolute path of the input MD file.

def md_filepath(self):
        """Returns (at runtime) the absolute path of the input MD file."""
        if self.md_node is None: return "MD_FILE_DOES_NOT_EXIST"
        if isinstance(self.md_node, FileNode): return self.md_node.filepath

        path = self.md_node.outdir.has_abiext("MD")
        return path if path else "MD_FILE_DOES_NOT_EXIST"