+ Minor changes to externilze the property/plugin mechanism.

+ Minor changes to externilze the property/plugin mechanism.

   no need for dedicated plugin to create volumes
   as we can also use shapes
 - added optional positioning to the SubdetectorAssembly_geo
   plugin for allowing envelopes to be placed

   envelope for subdetectors by calling a plugin that
   creates a volume
 - added factory methods for creating a volume from 
   an XML tag - to be used for envelope volumes

No code changes. Common code formatting removing tabs, replacing with spaces (tab-size=-2) + emacs reformatting.