Current FreeSurfer LTA implementation conflates head-motion series with transform array

[oesteban]

The LTA format allows reference and moving to be defined per entry in the affine list. However, our current implementation interprets these as a list of affines all referenced to the same reference/moving pair.

It would be easy to fix FSLinearTransformArray class-methods to check whether the src and dst are constant and if not, generate a list of FSLinearTransform, keeping the FSLinearTransformArray otherwise.