AP_Baro: auto-set field elevation from origin

[tridge]

This sets the active baro field elevation from the origin altitude if not set by the user. This should make baro altitude better match aviation altitude for all users, without having to setup the parameter on each flights ping @rbeall tested successfully on a CubeOrange plane with 4 flights

[peterbarker]

@tridge this has reliably broken Tracker's SetHome test.

I'd place a small wager on it breaking vehicles with ARMING_REQUIRE 0 given the nature of one of the patches in here, too.