Read and write

Functions to read and write vreg volumes to disk.

write_npz(vol, filepath)	Write a volume to a single file in numpy's uncompressed .npz file format
write_nifti(vol, filepath)	Write volume to disk in NIfTI format.
read_npz(filepath)	Load a volume created by write_npz()
read_nifti(filepath)	Read volume from a NIfTI file on disk.