handle building-dwelling data discrepancy

Problem → Differences in the level column when comparing the building and dwelling "layers" after processing the BD-Topo database

origin → most likely, from the calculation of levels performed by the extract_dwelling() function. This calculation overwrites the level column coming from the building “layer”.

solution → refine the calculation either by estimating the case where the level column is empty, or by adding our level estimates to another column instead of overwriting the one in the building “layer”.