Spatial smoothing & disallowing ANTs' subsamplings/smoothings

[oesteban]

The first registration level (likely using the new FullDKIModel or similar) can substantially be sped up if we spatially smoothed & downsampled the input before both model fitting and image registration.

This will make ANTs' settings easier, as the registration pyramid is set up globally, not just for registration.

This is one of the ideas I partially stole from @tmspvn.

[arokem]

That's a 💯 idea!