Disparity map results in `nan` when focus_distance = 0

[nikhil-97]

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.