Change DeformationFieldTransform and PositionFieldTransform to use Compsite<T>

[cmhulbert]

@bogovicj let me know what you think. It's in draft for now, but I think I implemented everything in #41

[bogovicj]

I'd like to add some tests that check

• PositionFieldTransforms with numSourceDimensions() != numTargetDimensions()
• applying these transforms for points of higher dimensionality