Constructor that reads in a file in YAML format.

        Args:
            filename (str): Filename.

def from_file(cls, filename):
        """
        Constructor that reads in a file in YAML format.

        Args:
            filename (str): Filename.

        """
        yaml = YAML(typ="safe")
        with open(filename, "r") as f:
            d = yaml.load(f)
        return cls.from_dict(d)