Added DFE.
This release adds a DFE to the example Rx model.
Note: The example models are still Init()-only. In order to represent the action of a DFE in the impulse response returned from AMI_Init(), I used the ideas presented in Sec. 4.3.4 of the DesignCon 2008 paper, "Demonstration of SerDes Modeling using the Algorithmic Model Interface (AMI) Standard" by Mike Steinberger, Todd Westerhoff, and Christopher White.