This function is a shortcut which creates a marked point pattern
from a given point pattern X and a given vector m
by attaching the marks in m to the locations of the points
in X. If X is already a marked point pattern, then its marks
are ignored and replaced by the values in m.
If m is a single value, then all points will be
given this mark value. Otherwise, m must be a vector
of length equal to the number of points in X, and
the point X[i] will receive the mark m[i].
Use unmark to remove marks.
Use ppp to create point patterns in more general
situations.