1. Initial commit. Fix for non-grouped time-ranges with
   null timestamps, in ASC order.
2. Fixed non-grouped time-ranges with null timestampw, in DESC.
3. Fixed grouped time-ranges with null timestamp, in ASC.
4. Fixed grouped time-ranges with null timestamp, in DESC.
5. Fixed grouped time-ranges for ASC (following)
6. Tests for different combinations of timestamp null grouping,
   timestamp ordering, and key grouping.
7. Refactor: Common code gathered to utility function.
8. Code formatting

Added to CHANGELOG.md.

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1. Non-groupby case: Use null-count to calculate null bounds
2. Groupby case: Use partition_point()

1. Short circuit eval for all-null/no-null cases.
2. Test cases for all-null/no-null.
3. Code formatting

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1. Fixed angle-brackets for header inclusion.
2. Switched to doxygen-style function documentation

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3. Code formatting

1. Streamline null_bound calculation, with fewer comparisons.

2. Fix botched null_bound calculation cleanup.
3. Remove errant println in tests.