Add a const getCopy method to the SysCopyHandle and use it in the AsgSelectionAlg

We have cases where we would like to allow using a read-only container in the selection algorithms. This requires adding a method to CP::SysCopyHandle to use a const pointer. Longer term we could consider moving this to a CP::SysReadHandle and using a separate algorithm to create a shallow copy where necessary.

FAO @tadej, @krumnack