``Marks'' are observations attached to each point of a point pattern.
For example the chicago dataset contains
the locations of crimes, each crime location
being marked by the type of crime.
This function tests whether the point pattern X
contains or involves marked points, and that the
marks are a factor.
It is a method for the generic function
is.multitype.
The argument na.action determines what action will be taken
if the point pattern has a vector of marks but some or all of the
marks are NA. Options are "fatal" to cause a fatal
error; "warn" to issue a warning and then return TRUE;
and "ignore" to take no action except returning TRUE.