Convenient constructor to generates a POSCAR transmuter from a list of
        POSCAR filenames.

        Args:
            poscar_filenames: List of POSCAR filenames
            transformations: New transformations to be applied to all
                structures.
            extend_collection:
                Same meaning as in __init__.

def from_filenames(poscar_filenames, transformations=None,
                       extend_collection=False):
        """
        Convenient constructor to generates a POSCAR transmuter from a list of
        POSCAR filenames.

        Args:
            poscar_filenames: List of POSCAR filenames
            transformations: New transformations to be applied to all
                structures.
            extend_collection:
                Same meaning as in __init__.
        """
        tstructs = []
        for filename in poscar_filenames:
            with open(filename, "r") as f:
                tstructs.append(TransformedStructure
                                .from_poscar_string(f.read(), []))
        return StandardTransmuter(tstructs, transformations,
                                  extend_collection=extend_collection)