Multi-stage calculations

Cube provides rich [data modeling](https://cube.dev/docs/product/data-modeling/concepts) capabilities and supports [various use cases](https://cube.dev/docs/guides/recipes#data-modeling).

We would like to level up Cube's data modeling with the _post-aggregation engine_ that would allow for further manipulations with already aggregated data, supporting more sophisticated analytics use cases or providing a way to express them in the data model in a streamlined way:

- Period-to-date calculations, such as year-to-date (YTD), quarter-to-date (QTD), or month-to-date (MTD) analyses.
- Differences or changes: finding the difference between two aggregated measures, like year-over-year sales growth.
- Fixed vs. relative comparison, which are useful when you need to compare individual items to a broader dataset.
- Ratio and percent of total calculations, calculations of ratios or percent totals that need specific control over the numerator and the denominator.
- Segmentation and grouping, calculations of advanced segmentation or grouping that is independent of the view’s granularity.

It's currently planned for October 2024.

Out of scope:
- #8487