count geometries about vapour HOT 6 CLOSED

Bit more clarity:

• iterate all features and return FID values and geometry status (null?)
• consider geometry count (OGR can have >1)

Implement short-circuit versions after this base is available. (Currently the FID getter is using SQL from R)

It's not clear to me if feature-counting is particularly slower than is-geometry-null checking, but it's too dependent on driver to complicate this base level.

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Specifically, as per vapour_read_geometry_cpp

CharacterVector fid; 
////...
    poGeometry = poFeature->GetGeometryRef();
    fid[iFeature] = poFeature->GetFID(); 
    if (poGeometry == NULL) {
// etc.
    }

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Simpler is the scan the extents: #39

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vapour_geom_summary now includes FID, valid_geometry, and xmin/xmax/ymin/ymax

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Add a list of use-cases, to give some clues as to the best ways to do things. SQL is a hassle with the layer name, so limit_n and these extent tricks can be used for fun and profit

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limit_n is now applied across attributes and geometry reader (vapour_read_names uses SQL which is slow)

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