Add new modes to `IntervalListTools`.
- `INTERVAL_SUBDIVISION`, which is the default mode, and functions like the previous version of `IntervalListTools`.
- `BALANCING_WITHOUT_INTERVAL_SUBDIVISION`, which emits interval lists without splitting them up, and sizes the output interval list sizes to be no smaller than the largest input interval.
- Bug fix: don't emit empty intervals.
Merge pull request #6 from broadinstitute/mm_interval_list_tools_atomic_scattering

Add new modes to `IntervalListTools`.