Weka Core Implementation [Not Tested]

Weka Core is implemented but not tested. This includes:
* Attribute class;
* Instance class;
* Instances class;

Little changes and additions to the old classes:
* addition of getValueIndex in the Attribute class for returning the
index of specified attribute value, if the attribute is nominal;
* getAttributeIndex in the Instances class for returning the index of
the specified attribute in the dataset;
* addition of std:: everywhere where it was missing;