Add of a new type of Trajectory: SynchronizedTrajectory which is created
by the simulation of a synchronized model with an LHA.

Simulate function for SynchronizedModel executes and is tested by two
simple models.

Next step : Cosmos based tests.

Add of several useful functions for access values of model, lha and
trajectory objects.

an object.

Type stability of simulate, dist, read_trajectory seems to be preserved.

First implementation of LHA.
Synchronized simulation not yet tested.
Reading of trajectories with LHA seems to work with one test basted on
simulations that checks that the last value is well read.