save_segmentation_labels function in segmentation notebook is confusing

[cliu72]

Is your feature request related to a problem? Please describe.
In segmentation_utils, the save_segmentation_labels function is partly redundant. It is saving {fov}_segmentation_labels.tiff, which seems to be what is already saved as {fov}_feature_0.tif. In the segmentation notebook, the default for channels is None for this function, so a channel overlay with segmentation borders is not being saved by default. It is not obvious that channels should be set to ['nuclear_channel', 'membrane_channel'] to save these overlays.

Describe the solution you'd like
In the save_segmentation_labels function in segmentation_utils, remove redundant file saving. In the notebook, change the default channels to ['nuclear_channel', 'membrane_channel'].