YASim: implement a versioning system

YASim: implement a versioning system

user attribute "version" of the airplane element of the YASim config file
to define the version this config uses.
Example:
<airplane mass="1344" version="YASIM_VERSION_CURRENT">

Initially, the following can be used:
YASIM_VERSION_ORIGINAL - The original version of YASim as implemented up to
                         FlightGear 3.0.0
YASIM_VERSION_32 - The version of YASim implemented in FlightGear 3.2.x
                   (and the development version 3.1.x)
YASIM_VERSION_CURRENT - The current and latest version of YASim.