MC_Test fixed

The test plugin for 3Dviewer "Test Marching Cube" returns the following error: 

```
javax.media.j3d.CapabilityNotSetException: Group: no capability to detach BranchGroup
at javax.media.j3d.Group.removeChild(Group.java:225)
at MC_Test.displayCase(MC_Test.java:64)
at MC_Test.run(MC_Test.java:42)
at ij.IJ.runUserPlugIn(IJ.java:202)
at ij.IJ.runPlugIn(IJ.java:166)
at ij.Executer.runCommand(Executer.java:131)
at ij.Executer.run(Executer.java:64)
at java.lang.Thread.run(Thread.java:695)
```
The fix was made as follows: In the method ``displayCase``, removal of ``BranchGroups`` is now done only after checking:

1. If the child object is an instance of ``BranchGroup`` 
2. If the child object is detachable. 

After fixing the above problem and successfully running the plugin, there was another small bug that caused repetitive warning message "Case X already exists". This was because as added contents were never removed from the Universe, they collide with new content with the same name.   This was fixed by adding a line to remove all contents before adding a new Mesh content.