module-imu.cc

implements two uses of a IMU:

ModuleIMU ("imu") mimics an IMU by sending to the output
the measures of a strapdown IMU (angular velocity and acceleration
components in a reference frame attached to the chassis of the sensor),
making them also available as private data.

ModuleIMUConstraint ("imu constraint") prescribes the angular velocity
and the acceleration of the point it is attached to.

In both cases the IMU can be rigidly offset and oriented with respect
to the structural node it is attached to.

Test: in two separate terminals, execute

	mbdyn imu
	mbdyn imu_constraint

The model in file "imu" uses the "imu" user-defined element to produce
the output of a 3-axis strapdown accelerometer and gyro, and sends it
on a stream.

The model in file "imu_constraint" uses the "imu constraint" user-defined
element to read the output of the other model, and impose it to another body.

The motion of node 1 from "imu" should be (nearly) identical to that
of node 1 from "imu_constraint"

