Update Registration

[thewtex]

• [x] Exercises use %load
• [x] Add another registration example
• [x] Make sure that there is a good explanation for backward vs forward transform
• [x] Update cell 4 to point to updated slides

[bpaniagua]

@thewtex the tooth segmentation code will not be ready. Do you want to include the corpus callosum segmentation notebook here #4

[dzenanz]

@bpaniagua remove yourself from the assignees list if you don't want to get further updates.

[thewtex]

@bpaniagua yes, agreed. We'll punt on the tooth segmentation / registration for now.