Parameters

 Keyword  Position  Type       Description
 -------  --------  ----       -----------
    X        1      REAL       Centre of starting circle.
    Y        2      REAL           "          "
    RAD      3      REAL       Radius of circle (axis units).
    ITER     4      INTEGER    Number of iterations.
        Default = 1
    XCENT    -      REAL       Centroid centre (output).
    YCENT    -      REAL           "      "        "