S3 generic/method consistency

[jdmanton]

Currently, we have the following signatures:

xform:
  function(x, reg, ...)
xform.BrainTemplate:
  function(x, x2, data, transform, ...)

We cannot change xform() as this is part of nat, but we don't want reg as the whole point is to let the user avoid having to deal with registration files directly. This suggests that we should use a different function instead.

[jdmanton]

Fixed in 5c2efb9 by changing xform.BrainTemplate() to trans3d().