Rethink some of the header's default attribute values

[Enet4]

This was discussed in #40, but I'm filing an issue so that this concern is not forgotten.

• pixdim should be [1.0; 8] by default.
• srow_x and other should have an "identity" default.
• sform_code should be 1 by default IF it's really the normal way to encode the transformation. Maybe qform_code is the way, I don't know,

I believe we're still undecided on what should be the defaults for sform_code and qform_code.

[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

[jchoude]

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

[Enet4]

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