Merged refs/pull/5657/head from repository cms-sw
moved template instantiation out of eos files into CondFormats/Serialization lib, added usage of CondFormats/Serialization to BuildFiles who need the lib
activated template instantiations for polymorphic archive to resolve missing symbols
copied includes from classes.h to headers.h to avoid duplicate symbols on os-x
added explicit use of CondFormats/Serialization to avoid implicit usage
fix signature of virtual method in derived class (and make it const which it is)
Merge pull request #5689 from apfeiffer1/fixCondOnMac

Fixes for compilation issued in conditions on mac