Possibility for the user to choose the Force displacement export file name

How it is currently done: For now, the load, displacement, etc exported by the curve writer are written in a file named forceouverture.txt . This name is imposed. How it should be done: Add an optional parameter to eventually export these quantities in a file with a user defined name (but keep the forceouverture.txt name by default if the user does not mind about this file name).