Add multithreading capabilities

[dbaston]

The current Processor implementations (FeatureSequentialProcessor, RasterSequentialProcessor) could be extended to use multiple threads. (Alternatively, new Processor implementations could be added added). Some previous work was done by @shortcutman using TBB but this was before some internal API changes and the switch to C++17.

Care would need to be taken to avoid multiple threads reading from the same GDAL dataset.