KDM14 — Merge ClassUnit and InterfaceUnit

This issue is related to KDM14-20

Problem: During the code model generation for Java we want to identify which class are in fact interfaces. Problem is that we basically need to identify which classes are interfaces upfront or have the ability to convert KDM "class unit" to "interface unit". However, both the objects presenting these both inherit from datatype but at the java code level one can't convert (i.e.) cast from ClassUnit to InterfaceUnit.

Question: from an implementation point of view would it better to have a class object with an Interface attribute? For example the ClassUnit object has isAbstract. Can we implement isInterface to ClassUnit?

This change will make it easier to change the type of a given KDM object.

Note: Java 1.8 will start to allow code in interface specifications which will blur the lines between interface/class specifications.

Defer to KDM 1.5

This issue will have a fairly significant impact on implementation, so it requires additional cycle for discussion.