Add support for directing node-branch sends to PEs

[jszaday]

This PE adds support for directing sends to particular PEs for node-groups via CkEntryOptions::setNodeGroupPe and/or CkSendMsgNodeBranchPe. Example use-case shown here: https://gist.github.com/jszaday/486ee195180c42eed2a6f7b14462b123

This was made after an inquiry from Nils Deppe. Couple of general questions:

• Does this violate any properties of the Charm++ model? Is it too niche?
  • Sanjay: "But... this seems odd breach of abstraction. Nodegroups are meant for work that any pe on the node can do. I think a common pattern is to always have a group associated with a nodegroup.So using a group may be a better solution. No?"
• Does this functionality already exist and we missed it?

TODOs:

• [ ] Decide on names for all added methods.
• [ ] Decide whether we need support for inline/immediate sends via this new procedure?
• [ ] Update documentation.