def count value=false, &block
          if block_given?
            @data.select(&block).count
          elsif value
            count { |val| val == value }
          else
            size - indexes(*Daru::MISSING_VALUES).size
          end
        end
Retrieves number of cases which comply condition. If block given,
retrieves number of instances where block returns true. If other
values given, retrieves the frequency for this value. If no value
given, counts the number of non-nil elements in the Vector.