More user-friendly bundling

[bknueven]

PHBase.PHoptions["bundles_per_rank"] should be (optionally) calculated based on an easier-to-understand number like total number of bundles or bundle size.

[DLWoodruff]

I guess I am OK with the proposal, but we should bear in mind that our "users" are assumed to be programmers so they should be the ones to make it nice for the end-users. If we let our users supply other values, then our code can convert it to bundles per rank rather than theirs so I guess there is some social welfare gain.

Another thing to keep in mind is a desire on the part of some folks to generalize the notion of bundles so that they do not necessarily constitute a partition (e.g., some scenarios could appear in more than one bundle); not that "bundles_per_rank" is helpful for this, just something to keep in mind.