Added onboard local position estimate, no heading, for unit testing.

[jgoppert]

I think this is useful because it allows us to do px4flow development without attaching it to the pixhawk etc. This is also better for unit testing the px4flow.

Maybe some day we could have a flow sensor hooked up and run a unit test on each commit where it publishes its local position history while a servo moves it around.

[LorenzMeier]

Thanks! That sounds like a plan. It would actually not be that much of an effort with everything we have in place now.