Optimize GROUP BY in MariaDB ColumnStore

This project optimized the GROUP BY operator in MariaDB ColumnStore for workloads requiring disk-based aggregation. To achieve this a) unnecessary I/O operations in the aggregation algorithm were reduced, b) bugs in the previous implementation of disk-based aggregation were fixed and c) vectorized access to the internal hash map used for aggregation was tested. Additionally, Theresa fixed other bugs and improved documentation and the development setup, especially for new developers.