Add a feature to export individual columns from h5 data

[mattcieslak]

One cool thing about the h5 format is if you want to, you could "rehydrate" an scan and put it back into a volume. So if you wanted to spot-check a specific input file, but don't want to have to keep that file around, you should be able to use the group mask to write that individual data back into its original format.

Not sure what the command to do this should look like