ZBigArrays were omitted from being saved in notebooks in 5fb16acd
(erp5_data_notebook: Don't save ZBigArray in data notebook. It may be
too big that zope process may crash.).

However since ZBigArrays are regular ZODB objects with persistent
references to other ZODB objects there is no reason for us not to be able to
serialize them.

Add corresponding FIXME with link to more context.

/reviewed-by TrustMe