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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).