Add Altitude.msg

[ViktorCVS]

Description

Add a new scalar message to sensor_msgs named Altitude.msg, containing a timestamped vertical position z in meters (positive up), with an optional uncertainty. This fills the gap between existing messages that either (a) embed altitude together with latitude/longitude (NavSatFix) or (b) represent barometric pressure or generic ranges rather than altitude itself. The proposed definition mirrors existing scalar messages (e.g., Temperature, Illuminance) that carry a variance field.

Fixes #294

Is this user-facing behavior change?

No

Did you use Generative AI?

No

Additional Information

Intended publishers (examples)

• Barometric altimeters → reference=REFERENCE_DATUM, datum="MSL" or a specified geoid/ellipsoid (e.g., "EGM96", "WGS84").
• Underwater pressure sensors (depth) → reference=REFERENCE_SURFACE, datum="SEA_SURFACE"; report depth as negative altitude.

Before approving or merging, @tfoote requested additional files. I’ll address this as soon as possible, but I won’t have time over the next few weeks. You can see the request here