``Marks'' are observations attached to each point of a point pattern.
For example the longleaf dataset
contains the locations of trees, each tree being marked by its diameter;
the amacrine dataset gives the locations of cells
of two types (on/off) and the type of cell may be regarded as a mark attached
to the location of the cell.
This function tests whether the point pattern X
contains or involves marked points.
It is a method for the generic function is.marked.
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.