BattMon: rename Analog file
BattMon: add backend as friend class
BattMon: backend implements capacity_remaining_pct
BattMon: SMBus becomes child of backend
BattMon: fix SMBus_PX4 constructor to accept state
BattMon: fix SMBus_I2C constructor to accept state
Copter: remove smbus batt monitor

battery_monitor class instantiates smbus if necessary
Copter: correct sending of 2nd battery's voltage
Copter: use battery.has_current method
Copter: cli uses has_current method