A reference to the external file containing the structural models
in the ensemble. The number of models in the external file should
correspond to the number of models in the ensemble. This data item
is a pointer to _ihm_external_files.id in the IHM_EXTERNAL_FILES
It is recommended that the large ensemble files be stored as separate
zip files within the same DOI. It is also recommended that large sphere
model ensembles be in binary format, which facilitates faster access.
Currently, a binary dump of co-ordinates in dcd format is suggested.
The topology can be inferred from the IHM_SPHERE_OBJ_SITE and the
ATOM_SITE categories in the corresponding mmCIF file.