[Halld-offline] DSelector Now Works on MC-GEN Trees

[Paul Mattione]

If you make a tree of your generated MC data using the mcthrown_tree plugin, you can now analyze this tree with the DSelector.  Just update your gluex_root_analysis, make a new selector as before, and run over the tree as before.  

Note that it's not feature-complete yet, so give it a try and if there's something missing let me know. 

 - Paul