The function uniquemap is generic, with methods for
point patterns and data frames.
This function determines whether any points of x are
duplicated, and constructs a mapping of the indices of x
so that all duplicates are mapped to a unique representative index.
The result is an integer vector u such that
u[j] = i if
the points x[i] and x[j] are identical and
point i has been chosen as the unique representative.
The entry u[i] = i means either that point i is
unique, or that it has been chosen as the unique representative
of its equivalence class.