Add API for saving/retrieving Modulo objects

CoreMetadata internally keeps one Modulo object for each of Z, C, and T
(since that is all that the current schema supports).  These objects can
also be retrieved through IFormatReader's getModuloZ(), getModuloC(),
and getModuloT() methods.  All three Modulo objects will then be used
internally to populate ModuloAlong* annotations.