Handle Maidenhead grid squares

[ReneeVandervelde]

An alternative method of expressing a station’s location is to provide a
Maidenhead locator (grid square). There are four ways of doing this:
• In a Status Report — e.g. IO91SX/-
(/- represents the symbol for a “house”).
• In Mic-E Status Text — e.g. IO91SX/G
(/G indicates a “grid square”).
• In the Destination Address — e.g. IO91SX. (obsolete).
• In AX.25 beacon text, with the [ APRS Data Type Identifier — e.g.
[IO91SX] (obsolete).
Grid squares may be in 6-character form (as above) or in the shortened
4-character form (e.g. IO91).