Bug 181121 Model should provide utility method for cube to get correspond dimension key.
In model cube handle, the relation of cube data set and dimension data sets are managed by a series of dimension condition handles.We need model to add utility method so that we can easily get corresponding dimension data set column names according to given cube dimension keys.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/MemberRef.java
public ElementPropertyDefn getPropDefn( )
	{
		return propDefn;
	}
	/**
	 * Returns the definition of the member to which the reference points.
	 * <p>
	 * property. <strong>member </strong> <br>
	 * property.member. <strong>member </strong> <br>
	 * property.member.list[n]. <strong>member </strong> <br>
	 * 
	 * property.list[n]. <strong>member </strong> <br>
	 * property.list[n].member.list[n]. <strong>member </strong> <br>
	 * 
	 * @return the definition of the target member
	 */
	public PropertyDefn getMemberDefn( )
	{
		return member[1] == null ? member[0] : member[1];
	}
	/**
	 * Returns the structure pointed to by the reference.
	 * <p>
	 * <strong>property </strong>[.member] <br>
	 * property. <strong>member </strong>.member <br>
	 * property.member. <strong>list[n] </strong>[.member] <br>
	 * 
	 * property. <strong>list[n] </strong>[.member] <br>
	 * property.list[n].member. <strong>list[n] </strong>[.member] <br>
	 * property.list[n]. <strong>member </strong>.member