This function creates an object which represents
a missing or unavailable object belonging to one of the
classes in the spatstat package family.
For example NAobject("ppp") creates an object which inherits
the classes "NAobject" and "ppp", and effectively
represents a point pattern dataset that is missing or unavailable.
Typical uses of NA objects include simulation algorithms (where the
algorithm may sometimes fail to produce a result) and designed
experiments (where data may be missing for some of the experimental
cases).
An NA object can be recognised by is.NAobject.
NA entries in a hyperframe can be recognised by
is.na.hyperframe.
NA entries in a spatial object list can be recognised by
is.na.solist.