Comment out set_Static_pressure, set_Static_temperature, and

Comment out set_Static_pressure, set_Static_temperature, and
set_Density; since JSBSim doesn't override the get_* methods from
FGInterface as well, the wrong values are always set anyway.

Take the static temperature, static pressure, and density values from
an external source if requested by the
/environment/params/control-fdm-atmosphere property.  This isn't
working properly yet for JSBSim because of interactions with the
trimming routine.