hysteresis in alarm levels

[moshe-braner]

Should allow repeated alarms for same traffic if it gets closer - but don't want constant alarms for same traffic. If a lower alarm level is required to re-arm the alarm, traffic that crosses the boundary back and forth will still cause unwanted repeated alarms.

[moshe-braner]

Modified source code to:

• in Legacy.h added ALARM_LEVEL_CLOSE between NONE and LOW
• in TrafficHelper.h added ALARM_ZONE_CLOSE distance between NONE and LOW, and similarly a time constant for vector method
• changed ALARM_ZONE_NONE to somewhat more than 10000 meters, to include farther traffic in data reports through NMEA etc
• remember previous alerted alarm level of each target
• sound a new alarm whenever the level increases by 2 over the previous alert level
• if current alarm level decreases, lower the future alert threshold.
• Also modified NMEA_Export() to return the exported alarm level codes to what they were, masking the added "CLOSE" level.

[moshe-braner]

Found a bug in version MB05 that prevented the hysteresis. (At time of Traffic_Update() of newly arrived packets from already-known traffic the historical alert_level was not yet filled in.) Will be fixed in MB06.