Track NUCp on positions

[mutability]

Derive it from the ME type.
Store the effective NUCp in aircraft state (min of even/odd for global CPR, min of ref + new for local CPR)
Maybe export it via JSON.

Will come in handy for reasonableness checks - if we get an "unreasonable" positions, comparing NUCp will tell us whether to discard the old position or the new position.