Adding inputs to demons with optional InitalDisplacementField

Naming the first 2 parameters and FixedImage and MovingImage. Adding
an optional InitialDisplacementField input.

Change-Id: I18bff984f4312324a60df76f0e3ed029d9a1d32b
Rename deformable registration to demons exmaple

Change-Id: I00d8ef906c1eb95c1579d9225f08a5cb21d3a9b9
Adding python example with initial transform

Adding an additional example for a demons registration which utilizes
the initial transform input in combination with the transform to
displacement filter.

Change-Id: I8117f6c6818c44a58588d66ebdc5330854f323ba
Adding DemonsRegistration2 for c++ example

Change-Id: Icef6a1d92a6fb766ee916e6e8feca5340450e882
Merge topic 'SIMPLEITK-541_AddInitialDisplacementFieldToDemons' into next

afe731b8 Adding DemonsRegistration2 for c++ example
f5f76cf1 Adding python example with initial transform
f06179a3 Rename deformable registration to demons exmaple
e256e6f7 Adding inputs to demons with optional InitalDisplacementField