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
switch ( refType )
		{
			case PROPERTY :
			case PROPERTY_MEMBER_LISTn :
				return struct;
			case PROPERTY_MEMBER :
				return struct.getProperty( module, member[0] );
			case PROPERTY_MEMBER_MEMBER :
			case PROPERTY_MEMBER_LISTn_MEMBER :
				return struct.getProperty( module, member[1] );
			default :
			{
				assert false;
				return null;
			}
		}
	}
	/**
	 * Gets the local value of the referenced property, structure, or member.
	 * 
	 * @param module
	 *            the module
	 * 
	 * @param element
	 *            the element for which to retrieve the value
	 * @return the retrieved value, which may be null