Use a single calculated table for date template

[Alberto Spelta]

Discussed in #638

^{Originally posted by bdarbonneau April 11, 2023}
When Bravo creates a calendar table, it creates a hidden calculated table, then another one that refers to the first one. The model eventually contains two identical tables, one of them not used but which still consumes memory. Why not create a single one ?
I am curious to understand the reason behind this design choice.