Python constructors from raw buffers

[kylebarron]

Change list

• Add from_parts constructors to the six core geometry array types. This allows constructing geometry arrays from raw coordinates plus offset arrays.

Todo:

• Allow passing in an optional null buffer
• Allow passing in array metadata (e.g. CRS)
• The chunked arrays already have a from_arrow_arrays method, so you should probably consolidate that and from_chunks.