Progress bar for 3d

[kwinkunks]

Reading a 3d will likely be slow — unless we figure out random file access, eg w segpy — so might need this trick. See, eg, http://www.lihaoyi.com/post/BuildyourownCommandLinewithANSIescapecodes.html

[kwinkunks]

This could be a way to do it, but unfortunately introduces another dependency...