[bug] RGB nifti encoding standard unclear

[xgui3783]

despite #15 , it seems different nifti softwares (e.g. fiji nifti plugin) seems to store rgb channel on 3rd index, rather than 4th index. related PR #23 is somewhat suppose to fix this, but it would be more suitable to clarify how RGB is encoded.

Unfortunately, this feature is outside of my capacity at the moment. Perhaps I can pick it up in a few months.