Disparity map results in `nan` when focus_distance = 0

There's an error in the function `render_depth_and_disp_maps(..)` in lightfield_simulator.py .
When the `LF.focus_dist` is set to 0, it results in the calculated `disp` becoming `nan` as it is divided by 0.
Better to perform a check before applying the formula, so that `LF.focus_dist` or `LF.sensor_size` are not set to zero.