nx.tcl

- nx.tcl: * new method requireClass to Combine two classes and return the more specialized one * make slot objects for parameter aliases and parameter forwarder instances of ObjectParameterSlot * get rid of attributes "isforward" and "isalias" and replace it by "dispositon" * complete list of predefined value checkers * we have now three approaches for providing parameter -object-filter and -object-method Approach 1: create RelationSlot with nosetter Approach 2: use parameter forwarder Approach 3: use parameter alias we switched from approach 1 to approach 3