Add function Gini(), to calculate Gini value

Input:
1. The attribute list that we need to calculate - ArrayList
2. The classified attribute list - ArrayList
3. The name of the attribute - String

Output:
Class Result
1. result.getGiniValue() : The GiniValue  —double
2. result.getAlMinLeftChild() : One sublist of best division
— ArrayList<String>
3. result.getAlMinRightChild() : The other sublist of best division
—ArrayList<String>