add: Example Python scripts for image registration

[schuhschuh]

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

• 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?