Add new height attribute `HHT` and adapt the function `is_valid_height_tag()` to handle `HHT` correctly.
To be able to use the total height of a building as provide from GHSL, we need to introduce the height tag HHT of the GEM Taxonomy describing the total height in meters. This requires also to adapt the function is_valid_height_tag() to also check for the attribute HHT and to handle it correctly.
Additionally, first tests for the modified functions should be implemented.