Separate building and building:parts to fix the overlapping building geometries

Buildings in OSM source are main buildings and the building parts, we process all of them so the output has overlapping geometries which makes the dataset unusable for some users.

Based on the discussion we had on #54, having a new category=2 for all the building:parts is the first go-to solution. The main building has the complete building outline and will stay as category=1. If the solution works in entity-initializer, the subsequent processing would not be affected a lot since we use category=1 as buildings.

\fyi @all