Added an interface for an environment controller, together with a

Added an interface for an environment controller, together with a
default implementation that uses user-supplied params.  Currently, the
only parameters are

  /environment/params/base-wind-speed-kt
  /environment/params/gust-wind-speed-kt

but others will show up soon (i.e. sheer, variable direction, variable
visibility, etc.).  To activate these properties, you have to
configure --with-new-environment.

The gusting function is simplistic and needs to be replaced with
something better, though it doesn't feel too far off.