Options can now be selected at
command line without requiring
separate builds of the code

greyscale, so nerf learns alpha
channel rather than rgb channels

Fixed bug in testbed code

Adjust python api to output slices from
an image so that this can be done from
the command line

Also saves updated up-dir to snapshots

Add NeRFCapture to docs

# Conflicts:
#	docs/nerf_dataset_tips.md