code: public int expandedLength() {
      if( _v1Domain==null && _v2Domain==null ) return 1; // 2 numeric columns -> 1 column
      else if( isCategorical() ) return domain().length; // 2 cat -> domains (limited) length
      else if( _v1Domain!=null ) return _v1Enums==null?_v1Domain.length - (_useAllFactorLevels?0:1):_v1Enums.length-(_useAllFactorLevels?0:1);
      else return _v2Enums==null?_v2Domain.length - (_useAllFactorLevels?0:1):_v2Enums.length - (_useAllFactorLevels?0:1);
    }
docs: Obtain the length of the expanded (i.e. one-hot expanded) interaction column.