AP_BattMonitor: added a "battery" backend for liquid fuel flow

[tridge]

This is for sensors that give a pulse for each fixed volume of fuel.

Output is:
- current in Amps maps to in litres/hour
- consumed mAh is in consumed millilitres
- fixed 1.0v voltage

Here is an example sensor suitable for this driver:
https://www.btflowmeter.com/en/flow-meter-products/flow-meters-lowflow-flowmeters-low-flow-turbine-flow-meter-fuel-flow-meters-diesel-fuel-flow-meter-watermeters-paddlewheel-flow-meter-oil-flow-meter-oilflowmeter-waterflowmeter-water-meters-turbineflowmeter-oilflowmeter/mini-flowmeter-fch-mini-pp-series-chemical/fch-m-pp-30-lpm-97478169-lc.html

Thanks to RFDesign for sponsoring the development of this driver!

[WickedShell]

Outside of the inline comments do you have a link to the actual/example sensor that this is adding support for?

[davidbuzz]

looks good, but I note that there's no way to zero the "consumed" amount other than a reboot, which isn't always a practical pre-flight solution with the engine already running. Eg, many fuel-based RC planes use a "taxi tank" or "startup tank" which keeps the in-board tanks full right up to the moment of departure, so it would be helpful to be able to zero the consumed amount/s. ( ie _state.consumed_wh and _state.consumed_mah are never reset back to zero anywhere, maybe a mavlink packet to do it? )

[tridge]

@davidbuzz yes, it should be part of the PREFLIGHT_CALIBRATION options