OpenAIR: Add additional command TP to provide transponder code

[reskume]

For certain airspaces, it would make sense to have an optional TP command that provides a transponder code, e.g. for a TMZ.

[lordfolken]

i´d second such a field.

[reskume]

With the next big feature update for OpenAIP (which will happen in the next weeks), we add support for adding transponder settings to each airspace. Each settings consists of code, primary flag and a remarks field. For an airspace with multiple settings, the first "primary" transponder setting will be exported into a TP command which will contain the code value.