Move to string-based log header operation

This might need Python and Flightplot changes, but saves onboard flash and/or RAM while still allowing quite large log messages with 30-40 floats (which will have a performance impact if logged at high rate, but that is not what we are expecting).

@DrTon Could you have a look? I haven't tested it yet, but it would be good to know what level of changes on FlightPlot you expect to be necessary.

I would assume that changing the format strings and labels is easy (and compatible in that the new parser can could parse old and new formats without difference), but we might want / need to retain the message name field.