Registrations should yield transform objects.

[nfaggian]

Cleanup model.py, interface for transforms should be mostly similar to that of skimage.

Need the basic properties:

1. .matrix
2. .vector
3. .inverse

In abstract base class?

4. mul(self, other):
   return tform(matrix=np.dot(self.matrix, other.matrix))

** keep concept of coordinates.