DAC-ADC ramp functionality needs revision

[zibrov-zlobin]

We have three independent functions - ramp, ramp2 and buffer_ramp and buffer_ramp_dis. Is there a reason to keep them separate?
I think we need just one that can have an optional list of input channels.
the input channels should be able to accept both a list and a single value.
Its makes no sense to write buffer_ramp([1], [0], ...) to ramp and read a single channel.