Add shape-update-method option

[gdevenyi]

There are several potential ways to do shape updating, we should provide an option for each

1. ANTs method -- average forward transforms, generate "scaled pseudo inverse" and apply 4x to the intensity average
2. vfonov method -- average all forward transforms, estimate inverse of average (possibly scale), and compose along with individual forward transforms, then compute intensity average
3. "no update" method -- used by methods such as the original MNI construction, volgenmodel et al, always forward register to the target, shape update is a no-op