This function recognises whether an object is
a missing or unavailable object belonging to one of the
classes in the spatstat package family.
In spatstat, a missing or unavailable object of class
"foo" is represented by an object that inherits
the classes "foo" and "NAobject".
For any object x, the command
is.NAobject(x) will return TRUE if
x is a missing or unavailable object, and FALSE otherwise.