objects that are supplied as arguments are averaged in a way that
makes sense for the object class, i.e. taking into account
the particular bad-data codes of that particular class.
either a list of oce objects, in
which case this is the only argument, or a single oce object,
in which case at least one other argument (an object of the same size)
must be supplied.
object is a list. Otherwise, one or more
oce objects of the same sub-class as the first argument.