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

This patch uses all derivative values from qss 1, 2 or 3 to produce
better output in file and plot. A new file is available (discrete file).
It provides a discrete time output (using a new dt parameter) while the
old only stores all derivative values. (Closes: #32).

An fix many open/close file. (closes: #28).

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.