Implement NodeProxy as default objgroup for bare handlers

[lifflander]

What Needs to be Done?

Currently, objgroups go through a different API and always have an object associated with them. While for many users objgroups are ideal sometimes a bare handler is needed or more useful. Much of vt is implemented using bare handlers.

Create a special objgroup proxy instance (and maybe a specialization with separate logic?) that sends messages to bare handlers using a symmetrical proxy interface.

[lifflander]

@jstrzebonski Here's the next issue I would like you to work on.

Right now, to send a message, you call theMsg()->sendMsg<...>(rank, msg);. The objgroup API allows you to send messages to an object group instance (always one per node) doing proxy[I].send<>(msg).

I want to the keep the active message send API as is, but add an alternative way to call it. Like default_proxy[I].send<>(..) that sends to the node without an object group, but just calls down to theMsg()->sendMsg.

Let me know if you have any questions.