The current vertical division setting (per channel) and the timebase are stored
as a floating point number. This is suboptimal since clients expect us to send
this information to them in form of a rational number.
Store only the index of the current setting since all the supported settings are
already stored inside of an array.
The DF_END packet was send out after all configured frames were fetched, but
devices may stop the acquisition at any point in time and an DF_END will not be
send out in this case.
Send the DF_END packet inside of acquisition_stop() so this can't happen.