Support SimGear logging

[legoboyvdlp]

When JSBSim is used with FlightGear, the std::cout / std::cerr messages are printed to the console. FlightGear closes so quickly, that it is impossible to read the error messages.

If JSBSim would use the SimGear logging mechanism, this would allow the error message to be routed to fgfs.log, so that the developer can read the message. Otherwise, the error message is not useful, because it is not possible to read it before FlightGear -- and the console -- close down.

As requested on sourceforge ticket.

[bcoconni]

Thanks for the feature request @legoboyvdlp. I guess indeed JSBSim could make use of SG_LOG to redirect the messages to various locations (console, log file, etc.).

We need to see how fonts decorations (colors, bold, etc.) are managed since they are using control characters which will look ugly on a log file.