FOVsplitter improvements

[luigiferiani]

split_fov

• Stricter bounds for wells lattice placement
• Flag is_dubious in /fov_wells's _v_attrs to check whether a FOV splitting is not reliable (thresholding the output value of the fitting function)
• Tierpsy will print a message when encountering a is_dubious splitting
• Solved a possible out-of-bounds bug in tile_FOV_2D and tile_FOV_3D
• begun to pep8ify FOVMultiWellsSplitter.py
• plot_wells now can work on an existing axis
• calculate_bgnd_from_masked_fulldata from split_fov/helper changed name to get_bgnd_from_masked as it's a better fit