changes in pico_adc:
- rewrite code keeping old interface
- modify get_info command: print more information
- return +/-Inf at overflow
- allow 1-digit channels in chan_set command
- check timing configuration in get command
- new command: set_dig_out, setting digital output
- new command: disable_all, reset all channel settings
- new command: get_val, read a single value, no channel setup needed
- update documentation