Multithreading for hungry processes

[onceadrog]

Some analysis processes like DirectSunHours are pretty hungry with more complex models, but still only use a single core, and show as unresponsive despite still working away nicely.

Is multithreading viable? And keeping alive so Windows doesn't want to kill it?

[Moult]

Python in Blender generally is happiest when it is using a single core. Ideally, the multicore is split off into a completely separate system (such as a SWIG-accessed C++ function), which would be provided by upstream.

Ping @mostaphaRoudsari @chriswmackey