Add OpenBuildings dataset

[nilsleh]

This PR closes #68 and adds the OpenBuildings dataset.

Dataset features:

• more than 516 M building detections across the African continent
• confidence score per building detection
• google Plus code corresponding to polygon building center

Dataset format:

• building data comes in 135 csv.gz files
• meta data file tiles.json that has geometry of each of the 130 single tile
• 5 csv.gz files are not covered in the meta data file since they are very small I suppose (one entry)

Questions/Concerns:

• I could not find the source crs
• Currently, loading an item can take fairly long because some csv.gz contain millions of buildings
• It is quiet possible that a BoundingBox query returns a hit on a file but still finds no shapes for this area because the tiles are so large.

Thus, I would welcome suggestions on these points, as well as any other suggestions of course.

Plot example:

[adamjstewart]

I could not find the source crs

Can you elaborate on this? If there is no CRS, then there's no way to make this a GeoDataset, right? You have to have some transform to map this dataset from one CRS to another in order to use it geospatially.

[nilsleh]

Can you elaborate on this? If there is no CRS, then there's no way to make this a GeoDataset, right? You have to have some transform to map this dataset from one CRS to another in order to use it geospatially.

I could not find a crs in the metadata file or the csv files where the polygon strings are. However, I assumed that since there are geographical coordinates, there would also be a CRS. But since it is not the case, I was wondering if there is maybe another way to find out or to handle it?

[adamjstewart]

Hmm, if the coordinates range from [-90, 90] and [-180, 180] or [0, 360] then we can probably assume that things are simply in lat/long units?