Allow custom type aggregations

[gregmacfarlane]

The floor_types and other included lookup tables are handy, but we should have an override field that would allow us to use custom aggregations. This might work as follows:

extracting_thing(db, ..., floor_types = NULL, ...){
  if(is.null(floor_types)){
    #included floor_types
  } else {
    # use custom lookup
  }
}