def count_values(*values)
      values.map { |v| @cat_hash[v].size if @cat_hash.include? v }
            .compact
            .inject(0, :+)
    end
Count the number of values specified
@param [Array] values to count for
@return [Integer] the number of times the values mentioned occurs
@example
dv = Daru::Vector.new [1, 2, 1, 2, 3, 4, nil, nil]
dv.count_values nil
# => 2