first complete example added

added a complete working example (the Hylemonella example with download links to the original tomogram)

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.

initial release of the first tutorial

A tomogram of Bdellovibrio bacteriovorus was segmented as the first (and currently only) example. (https://etdb.caltech.edu database ID mka2021-08-18-149). This example will probably be heavily modified and more examples will eventually follow.