NBX should be removed from sub index map creation

[jpdean]

A call to NBX was added in this PR to fix a bug in sub map creation, however, it should be possible to compute the sub map destination ranks without it by adding an extra communication step

[garth-wells]

@jpdean can this be fixed easily?

[jpdean]

Getting it to work shouldn't be too tricky, but doing it in a tidy, efficient way will need some thought. The main difficulty is that it's easy to overlook corner cases. I'll have a look at it this week.