#95 Aerosurface_scale documentation and code don't match

[Dennis Linse]

[Obviously low priority because it is working in all standard cases. Mostly "for the record"]

I noticed that the aerosurface_scale documentation in FGGain.h indicates that various min/max values default to values (implying you could have only a min or only a max) while the code indicates that things are all or nothing with domain and range min and max.

JSBSim manual (March 2010) isn't as free with defaults as the FGGain.h documentation.

Also the code in FGGain.c throws an error if the there is no <range> but does an exit(-1) if min and max are not present in the <range>.</range></range>