STYLE: Make `PixelTypeChangeCommand` local to ComputeSpatialJacobian()
STYLE: Make `PixelTypeChangeCommand` local to ComputeSpatialJacobian()

`PixelTypeChangeCommand` is just a hack, apparently, and it is only used inside `elx::TransformBase::ComputeSpatialJacobian()`.

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix