WIP: handle 2D XRFCT data

This MR aims at adding 2D XRFCT data handling to the nxtomomill fluo2nx converter.

• Add a dimension argument to the app.
• Add the key in the config.
• Modify from_fluo_to_nx() function to properly handle this.
• Add test for 2D.
• Modifiy 3D tests.