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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).

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).

The input widget does not affect the default_* variable used to
initialize or reinitialize models. This patch fix the variable.

(closes:#27).

Avoid clear observation vectors (size = 0, capacity = 1024 default)
during clear node editor function to reuse this vector.