unu join: Connect slices and/or slabs into a bigger nrrd. Can stich images
into volumes, or tile images side by side, or attach images onto volumes.
Usage: unu join -i <nin0 ...> -a <axis> [-incr] [-o <nout>]
-i <nin0 ...> = everything to be joined together (1 or more nrrds)
-a <axis> = axis to join along (int)
-incr = in situations where the join axis is among the axes of the
input nrrds, then this flag signifies that the join axis
should be *inserted*, and the output dimension should be one
greater than input dimension. Without this flag, the nrrds are
joined side-by-side, along an existing axis.
-o <nout> = output nrrd (string); default: "-"
