Overload the mutable properties of a PortNamespace when exposing it

[sphuber]

When exposing the ports of a process one would expect that the receiving port namespace would not just inherit the ports but also the mutable properties of the donor PortNamespace, such as the default, valid_type, help, validator and required attribute. Currently this is not the case. Additionally, any newly created namespaces will just receive the default properties. It would be good if both the inherited properties as well as those of a newly created namespace could be overridden from the expose call itself.

[sphuber]

Fixed in PR #1635