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irt::model and irt::dynamics are too closed. We merge these structure
into irt::model.

The new observation system provides better outputs. It relies on all QSS
integrator values (X, u, mu and pu). Observers callback can compute
intermediate values instead of interpolation.

This patchs changes the behaviour of observation function in QSS
integrator. Now, these function returns value, u, mu and pu value if
available. Observer callback must compute the values but can produce
better outputs.

This patch change the size of real array from 3 to 4 doubles. This patch
remove the length attribute. Now the size computed from the no-zero
value.

There is missing call to the finalize callback in observation system.
This patch fix the problem and provides new observation callback systems
based on function_ref. (Closes: #29).

Adding two global inline function pointers to allocate/deallocate
memory. This patch allows the use of external tools instead of default
malloc/free functions.