Associations with Java classes are handled inconsistently by CD2Pojo

[schmalzing]

Issue description

With active class2mc, Java classes can be used as source and target types of associations in class diagrams.

CD2Pojo currently handles associations with a CD class as source and a Java class as target, but silently ignores associations whose source is a Java class.

Reproduction

Example CD with a Java class as association source:

package test;

classdiagram CD1 {
  association Class -> Class;
}

Actual behavior

Associations whose source is a Java class are silently ignored. No generated field, accessor, or other Java representation is created.

Associations whose source is a CD class and whose target is a Java class are considered during generation.

Expected behavior

Java classes should not be permitted as either source or target of an association in a class diagram.

A context condition should reject both examples with a clear error message. Java classes may still be used as attribute types.