Group by arbitrary column

Currently, QuasarDB only supports grouping by timespans (e.g. group by 5min or 1h). This should be extended into grouping by arbitrary columns, so that queries like this are possible:

select max(bid) from in range(2010, 2020) group by 5min, exchange

The current workaround would be to create different timeseries per exchange, which is less flexible when you want to query the same dataset in multiple ways.