1. Enabled integral types. Some tests.
2. Fixed unsigned.
3. Tonnes of tests for nulls in orderby column
4. Purged references to timestamps in range-window code.
5. Tests for time scaling.
6. Added tests for mean. Fixed sum test.
7. Added collect tests.
8. Tests for ASC/DESC timestamps, all aggs.
9. Docs.
10.Renamed the orderby null tests.
11.Added range_window_bounds.hpp to meta.yaml.
12.Review comments (std::move, CMakeLists)
13.Better example for numeric range window function.
14.Removed TODO, again.
15.Formatting
16.Added integral range tests.
17.Removed scaling. Streamlined the dispatch logic.

1. Fixed @copydoc.
2. Added missing docs for range_window_bounds.
3. Removed invariant. Inlined into constructor.

1. Fixed punctuation.
2. Renamed add_and_check_overflow(), etc.
3. Changed range_window_bounds default to unbounded.
4. Removed test helper. Not reqd with C++17.

1. Use streams for calculating null-bounds.

1. Explicit conversion from timestamp type to duration type.

1. Cleaned up factory implementation, to make shared_ptr more apparent.
2. Documentation

1. Corrected docs for resolution of durations/timestamps.
2. Fixed nitpick in motor-racing dataset.
3. Switched bounds factory impl to use unique_ptr.
4. assert_non_negative() now uses if constexpr.