Added a new turbulence type ttStandard, and made it the default.

Added a new turbulence type ttStandard, and made it the default.

ttStandard is copied from ttBerndt, with the following modifications:

1. All turbulence is diminished within three wingspans of the ground.

2. The horizontal forces are used to calculate the moments, but then
   zeroed out so that only the vertical force is actually applied to
   the aircraft.

3. The yaw moment is not used.

In fact, the horizontal forces and the yaw moment should be allowed,
but they are extremely rare compared to the vertical force and the
pitch/roll moments.  For now, simply zeroing them gives the most
accurate feel.