fix method 2 and 3 responsibilities

[stefanches7]

should do the trick for #26

[Tokazama]

Thanks for the PR! , but we'll need something more involved in the future to fix the entire sform/qform situation here. The short explanation is that eventually we want to have both qform and sform, atleast when writing to a file. However, the NIfTI standard makes this a bit more complicated than moving between an affine mapping and quaternion.

If this is solves #26 then I think this is a probably a good enough fix for now. @saurvs, does this fix your use case? Could we have some sort of tests to ensure we don't break this in the future?