add: Example Python scripts for image registration

[schuhschuh]

For now, Instead of documenting all options, provide at least some example scripts...

• [x] Symmetric SVFFD registration script, conversion to SVF
• [ ] Inverse consistent registration using SVFFD model
• [ ] Pairwise FFD registration with bending energy, volume preservation, topology preservation
• [ ] Example of sparse 4D FFD of Whenze's paper
• [ ] Motion estimation given image sequence
• [ ] Motion estimation given image sequence and surfaces (e.g., Alister's airway motion)
• [ ] Others?