Added VisAttr ref extension compact construction

 - Using `ref="OtherVisName"` attribute, the visualization attribute is an extension of
the `"OtherVisName"` which is used to initialize the new vis attribute.
 - The new VisAttr inherits all the properties  of the ref and additional arguments override these values.

Example where the only difference is the `alpha` value.

```
  <vis name="SiVertexBarrelModuleVis"
       alpha="1.0" r="1.0" g="0.75" b="0.76"
       drawingStyle="wireframe"
       showDaughters="false"
       visible="true"/>

  <vis name="SiVertexEndcapModuleVis"
       ref="SiVertexBarrelModuleVis"
       alpha="0.5"/>
```

	modified:   src/plugins/Compact2Objects.cpp