rPref 0.6
=========

* Fixed bug for iterated application of partial evaluation (eval.pref) 

* Added partial evaluation functionality (additional parameter df) 
  to base preferences (base_pref)

* New default values for parameters of preference algorithms 
  (Scalagon alpha = 1, parallel computation is inactive by default)

* New plotting function plot_btg for drawing preference graphs
  
* Additional parameter flip.edges for plotting preference graphs

* Additional special variable "df__" in base preference to access the data set

* Added as.expression() function for preference objects
