Allow ITK DisplacementFieldTransform as input to library interface

[N-Dekker]

ITK's DisplacementFieldTransform should probably be converted internally to an elastix DeformationFieldTransform, which internally has a DeformationFieldInterpolatingTransform

Requested by Konstantinos (@ntatsisk)

[N-Dekker]

Should be addressed by supporting external transforms, by the following pull requests:

• Add "ExternalTransform" component and ElastixRegistrationMethod::SetExternalInitialTransform #944
• ENH: Write external initial transforms to ITK transform file #946
• ENH: Add SetExternalTransform to TransformixFilter #948
• ElastixRegistrationMethod.ConvertToItkTransform support external initial transforms  #949