The simulator flies very badly when using the full acceleration
code. I don't yet know if this is a bug in the simulation or a problem
with AHRS not handling acceleration for multicopters.
For now set the acceleration to be half a 'pure gravity' acceleration
and half from the full 3D calculation.
this re-works the multicopter simulator in terms of rotation matrices,
and adds full acceleration support, which means it will include linear
acceleration affects and centripetal acceleration
Andrew Tridgell