AthLinks: Implement thin() for GenericElementLinkBase.

This way, all ELs will have a thin() method. If thinning can't actually be supported (the index is not integral for example) then we give a runtime error. thin() should not actually be called in those cases, but it's needed for the compilation of some virtual functions.

Also update ElementLink documentation to say that thin() is required of the base class, and add a proper test for ElementLinkBase::thin().