GitLab

* refactor ssg_init_config and ssg_init_mpi to reuse common
internal init routine
* update both init routines to resolve self rank and HG address
before returning (this should obviate the need to ever call
ssg_resolve_ranks)
* update test programs according to above

Margo additions

I beefed up the margo-related part of ssg, including a "barrier" call (with naive implementation). I also tweaked the example programs. @ssnyder, mind taking a quick look at the API and see if anything's goofy?

Also, I changed the name of ssg to "Simple Static Group" here - it will go into master one way or another.

See merge request !1