Symmetrize objects after AMU to PAM50 registration

[jcohenadad]

Following registration (#40), the output mask is not symmetrized.

Possible solutions:

• symmetrizing the warping field.
  • pros: more elegant-- all objects from AMU could then be ported to the PAM50
  • cons: complicated, because we will need to act on an intermediate warping field (eg: step=1, not step=0).
• symmetrize the objects: T2s, GM mask
  • pros: simpler.

[jcohenadad]

Note: if you notice symmetry is not perfect, don't freak out-- it is a glitch in FSLeyes (notice the values in blue don't change):

[jcohenadad]

Fixed in bf872d6