Add pbv3_field command.

Useful for changing AMACv2 settings without having to look up the register map.

Usage: ./bin/pbv3_field [options] command [command parameters]

commands
 list: list available fields
 read field: read current value of field
 write field value: write new value to field

List of options:
 -b, --board       Powerboard number (default: 0)
 -d, --debug       Enable more verbose printout