QNH tracking

[19wintersp]

Tracks last received QNH per aircraft. Controller can see the last received QNH and whether it is the latest (via new tag item 451), update to the latest QNH (tag function 551), or reset the tracker (552).

On the code: QNH is tracked by a new singleton class which stores each QNH entry as a combination of QNH value, as the two least-significant ASCII digits of the pressure value, and a serial value, to mark a received pressure as stale if the aerodrome QNH changes to a new value, then changes back. The serial will overflow after about a year of continuous half-hourly updates. It probably would be nicer to use a packed representation for the QNH (eg. a short containing BCD values) instead of a pair of chars, but the chars can be copied directly from the METAR and to the tag item string, and the memcpy will probably be optimised to a register copy anyway. The QNH is extracted from the METAR on the assumption that the first occurrence of the string Q in the METAR string precedes the QNH, which is true of any valid METAR under Annex 3 (Q alone does not suffice because of the SQ condition).

Will look to test this at some point when I have time to do so

[19wintersp]

Minor issues worth considering

[19wintersp]

Seems to work fine