add UnitTest to check behavior when Safe = False

Check the behavior of all the classes of curves when evaluated at a time outside of the definition range with the parameter Safe = False.

It should not raise any error nor return undefined value. It should either :

• Evaluate the curve at the given time, even if it's outside the given interval, if it make sense mathematically
• Return the final/initial value of the curve if it's outside the mathematical definition interval.