DOC: Update texture feature class references.

The classes were refactored as described in

  http://itk.org/Wiki/Proposals:Refactoring_Statistics_Framework_2007_Migration_Users_Guide

Change-Id: I34fdc3f6c7f98dcf434404093d8d245e78c2d119
BUG: Fix texture feature correlation computation for constant image.

Prevent returning NaN.

ITK-3081

Change-Id: Ibcdf39c9183be21fb0b2c3f47b1cdde029ce588e
Merge topic 'TextureFeaturesNaN'

c85c9475 BUG: Fix texture feature correlation computation for constant image.
358d7c11 DOC: Update texture feature class references.