These functions decide whether the given line segments intersect
each other.
If A and B are two spatial patterns of line segments,
test.crossing.psp(A, B)
returns a logical matrix in which the entry on row i, column
j is equal to TRUE if segment A[i]
has an intersection with segment B[j].
If A is a pattern of line segments, test.selfcross.psp(A)
returns a symmetric logical matrix in which the entry on row i, column
j is equal to TRUE if segment A[i]
has an intersection with segment A[j].