nd-safir.py: add support for non-mrc files.

* is_imsubs(): new function to find if file is an mrc file of Priism.
* any2imsubs(): new function to convert from ImageWrapper into an
mrc of Imsubs (Priism).
* get_mrc_file(): plug in any2imsubs() in case of a non-mrc image.
* get_ndsafir_args(): make it more obvious why we would need the
"else: continue" otherwise we would add empty args to the system call.