`enqueue_write_buffer` and `enqueue_read_buffer` arguments

[jkozdon]

I was wondering as the reason for enqueue_write_buffer and enqueue_read_buffer taking different arguments, i.e., why does enqueue_write_buffer take nbytes and enqueue_read_buffer not. It seems that these should be same.

Also, currently nbytes is not used by enqueue_write_buffer, so the user does not have access to the low-level interface (instead the number of bytes is determined from the size of the host buffer).