function GetIndex(const Missing: string): integer;
Maps a value of a discrete attribute to its indexed (integer) representation and returns that index.
Returns: integer
Remarks:
All discrete values, which already occured once, are stored in the AttributeValueList property. If the AttributeValueList does not contain the current value of the attribute, the new value is added to the attributeValueList and a new index is assigned to it. The list is not sorted and it matters in which order the different discrete values are found. The Missing parameter is the value denoting a missing value. If the value of the attribute equals the missing value, the function returns -1.