Updated castor geometry seperated halves v2

This is an update to:
https://github.com/cms-sw/cmssw/pull/2722

We were asked to provide backward compatibility to old geometry. The numbering scheme now detects whether it's new or old geometry by looking for CASTFar/Near volumes which only exist in the (new) separated half geometry. With this change (even if old xml file or db is selected) no warning messages appear and det IDs are properly returned.
We can discuss this in simulation meeting on Friday...

(Including Hans @hvanhaev)