This is a clean up commit prior to the subsystem API standardisation to simplify
the diffs. It includes all SGSubsystem and SGSubsystemGroup derived classes.
Wake computations are now performed for all AI aircrafts within a range lower or equal to the value indicated by the property /fdm/ai-wake/max-radius-nm.
These computations are triggered by the property /fdm/ai-wake/enabled (it is disabled by default).
The result of the wake computations is not yet used by the FDMs so do not expect the user aircraft to react to the AI wake.
This is too destructive for many established things, so revert it.
Instead, make the FDM shell save /fdm state on postinit(), and
restore it on re-init (which reposition invokes).
As part of this, kill off the evil global FDM state, and avoid us
copying FGInterfaces (which is bad since it contains TiedProperties
and an FGGroundCache, neither of which are especially keen on being
copied). Will probably disable copy/assignment on TiedProperties in
a future commit.
