Support Altitudes in comments

[ReneeVandervelde]

Altitude may be expressed in two ways:
• In the comment text.
• In Mic-E format.
Altitude in Comment Text — The comment may contain an altitude value,
in the form /A=aaaaaa, where aaaaaa is the altitude in feet. For example:
/A=001234. The altitude may appear anywhere in the comment.
Altitude in Mic-E format — The optional Mic-E status field can contain
altitude data. See Chapter 10: Mic-E Data Format.