Implementation of FGWorldPartitionRuntimeSpatialHash.cpp required to cook WP-enabled levels

[Th3Fanbus]

https://discord.com/channels/555424930502541343/601030071221878784/1231161756039249951
https://discord.com/channels/555424930502541343/1036634533077979146/1292554180510220308

I tried to implement the class and got it working, but it would be best to have the real thing. Attached my implementation for reference:

FGWorldPartitionRuntimeSpatialHash.cpp.txt