swatch ticket 807

https://svnweb.cern.ch/trac/cactus/ticket/807
 - [x] writing tests for all below (first!)
 - [x] reading crate information in the SystemCreator? and using it to construct crates
 - [x] add System::add(Crate)
 - [x] add Crate::isSlotTaken(slot)
 - [x] refactor System::add(Processor) and System::add(AMC13Service) to take the above into account 
 - [x] make sure NULL pointers in System::add raise an exception