Added VisAttr ref extension compact construction

[whit2333]

Closes #841

BEGINRELEASENOTES

• Using ref="OtherVisName" attribute with the vis tag, the visualization attribute is an extension of
  "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"/>

ENDRELEASENOTES

[whit2333]

@MarkusFrankATcernch, can you have a look at this PR? I'm not sure if the checks fail because of something here.

[MarkusFrankATcernch]

@whit2333
This implementation will not work as expected.
It will apply defaults for all properties, which are not part of the XML definition.
I have implemented the stuff already some time ago, but did not push.
If you agree I commit my version and close this PR.
Otherwise you have to NOT apply all defaults if the "ref" property is present. Then I discard my changes.

[whit2333]

@MarkusFrankATcernch , You are right. I did push changes.

[whit2333]

@MarkusFrankATcernch, I have no problem if you prefer your implementation which does the same thing. The sooner it can be merged into the better!

[MarkusFrankATcernch]

@andresailer Please merge this PR. Thanks!