elimCloseCoord: Eliminate close (overlapping) points (in x & y space)
Description
elimCloseCoord reduces number of rows in 'dat' by eliminating lines where x & y coordinates (columns of matrix 'dat' defined by 'useCol') are identical (overlay points) or very close.
The stringency for 'close' values may be fine-tuned using nDig), this function uses internally firstOfRepeated.