documented use of bounding box as unitcell, added to mmtf
documented use of bounding box as unitcell, added to mmtf

fredludlow/ngl,arose/ngl,arose/ngl,arose/ngl,fredludlow/ngl,fredludlow/ngl,j0kaso/ngl,j0kaso/ngl,fredludlow/ngl,arose/ngl,j0kaso/ngl