[grid] support band scales. fixes #268

[hshoff]

🚀 Enhancements

• [grid] add support for band scales. fixes linearBand doesn't play with GridColumn #268

📝 Documentation

• [demo] demo band scale grid on /barstack

can also change the offset:

<Grid
  top={margin.top}
  left={margin.left}
  xScale={xScale}
  yScale={yScale}
  width={xMax}
  height={yMax}
  stroke={'black'}
  strokeOpacity={0.1}
  xOffset={(xScale.bandwidth() - xScale.step()) / 2}
/>