Fixed operator+ related to Property

There were 2 of them : one internal to add Property to Property, and one external to add any T to Property. Of course T could be Property too, creating errors like:

use of overloaded operator '+' is ambiguous (with operand types 'const Gaudi::Propertystd::string' (aka 'const Property<basic_string>') and 'const Gaudi::Propertystd::string' (aka 'const Property<basic_string>'))

This is now fixed by requiring that PropertyBase is not base of T.