feature request: smu cli: add voltage and current setting capability

[evils]

it'd be quite nice if the smu command line utility had, besides the ability to stream out samples: the ability to set output voltage and current limits for each channel

this would make the board a lot more useful (as someone who is reluctant to use python) as a small desktop power supply/sink that could e.g.: charge and discharge batteries and with very little shell scripting, measure battery capacity

side note a likely more complicated idea is to accept a stream from stdin to adjust these settings and a suggestion probably more suitable for the firmware repo: present the DACs and ADCs as stereo usb audio interfaces