First level verification for dcm2toad output

[cbedetti]

It would be nice if unf2toad could check if the right files have been created at the end of the conversion.

Right now, if the user converts fieldmap images (phase and magnitude) and fails to specify the right directories, only one image is created with no error.

I put first level in the title since the verification is only on the filename. The second level could be on the file type and/or format itself.

[arnaudbore]

Maybe we should check and cross more informations (ex: bvecs -> DWI ) to confirm good conversion.