Determine BufferGeometry attribute numItems automatically.

Setting the `numItems` property of attributes has always felt redundant to me as the information is already in `array.length`. I understand that the array may be deleted, so `numItems` must be used, but I can't see any reason why that couldn't be automatically set during init. However, should there be some obscure reason to have different `numItems` and array size, this PR preserves the ability to set it manually.