Add an empty band to a VRT_x object

Usage

vrt_move_band(x, band_idx, after, save_dir)

x: A VRT_x object
band_idx: numeric indicating the band number of the band to move.
after: numeric indicating the band after which the new band should be places. Note this is based on the initial state of the band ordering. eg. do not add 1 if you are moving the band forward.
save_dir: A character string indicating the directory to save the modified VRT. Defaults to the cache directory.