Added Ennis citation to class documentation

Author: Erik Ziegler

nipype/interfaces/dipy/tensors.py

@@ -41,6 +41,11 @@ class TensorMode(BaseInterface):
     b-vectors. Fits the diffusion tensors and calculates tensor mode
     with Dipy.
 
+    .. [1] Daniel B. Ennis and G. Kindlmann, "Orthogonal Tensor
+        Invariants and the Analysis of Diffusion Tensor Magnetic Resonance
+        Images", Magnetic Resonance in Medicine, vol. 55, no. 1, pp. 136-146,
+        2006.
+
     Example
     -------