jewettaij/visfd_tutorials: updated "STEP_0" of the Bdellovibrio segmentation example

Earlier versions of the tutorial argued that everybody should use NAD filtering to improve image contrast before using VISFD. After more experimentation, I came to the conclusion that NAD filtering is not always helpful. So I edited the "STEP_0" file (which recommended NAD filtering), making it optional and hopefully clarifying when NAD filtering may be useful.