erp5_data_notebook: Add FIXME about ZBigArray not being serializable

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

bt5/erp5_data_notebook/ExtensionTemplateItem/portal_components/extension.erp5.JupyterCompile.py

@@ -34,6 +34,9 @@ well_known_unserializable_type_tuple = (ModuleType, Record)
 # ZBigArray may not be available
 try:
   from wendelin.bigarray.array_zodb import ZBigArray
+  # FIXME ZBigArrays are regular ZODB objects and must be serializable
+  # FIXME the bug is probably in CanSerialize()
+  # FIXME -> see https://lab.nexedi.com/nexedi/erp5/commit/5fb16acd#note_33582 for details
   well_known_unserializable_type_tuple = tuple(list(well_known_unserializable_type_tuple) + [ZBigArray])
 except ImportError:
   pass