[Request]: support read absolute altitude

[zhuscat]

Plugin

sensors_plus

Use case

to track user's absolute altitude changes

Proposal

https://developer.apple.com/documentation/coremotion/cmaltimeter/3746858-startabsolutealtitudeupdates

From Apple's documentation, CMAltimeter supports read absolute altitude. Is it appropriate to add this feature to sensors_plus

[aanas-sayed]

I'm in need of this. Would happy to implement it but need ideas on how to structure it as a barometer call already exists, so how to add this?

1. As a separate function? if so what should it be called? Since the previous is called Barometer, this could be called Altimeter?
2. As an additional field (altitude) alongside pressure. This may not work because startAbsoluteAltitudeUpdates and startRelativeAltitudeUpdates may not update at the same time.

For Android, it can be accomplished using PRESSURE_STANDARD_ATMOSPHERE as sea level approximation and getAltitude

@vbuberen @miquelbeltran - Would either of you be able to comment on this please?