Implement `setindex!` for geospatial data

[juliohm]

Currently our geospatial data types implement getindex to access the underlying table of attributes and geometries. We need the same high-level interface with setindex! to save users from having to learn and use Tables.jl explicitly.

[davibarreira]

The getindex seems to be implemented at GeoTables.jl. Is the setindex! also to be implemented there?

[juliohm]

That was the original idea, but we need to wait on Tables.jl as it currently doesn't provide a setindex API.

We also concluded that the geometry column needs to be immutable for type stability in downstream algorithms.