Rethink some of the header's default attribute values about nifti-rs

[jchoude]

Just a minor correction, @nilgoyette , the qform_code should also be 1. 0 means that it's not recorded.

[nilgoyette]

My colleague @jchoude tells me that we should do:

• reading
  • Normal case: if either sform_code or qform_code is present (not 0), apply the requested one.
  • Special case: both are present. Standard says nothing about this case but most tools use sform
• writing: calculate and write the 2 transformations with sform_code = 1 and qform_code = 0. We will have to wait for #22 to do it properly though.

This doesn't directly answer the question, but as I understand it, the default Header should be

• "identity" in both transformations
• sform_code = 1
• qform_code = 0

[Enet4]

I believe this has been resolved in #56. As usual, this can be brought up again if something is worth rethinking.