dynamic `ThreadCount` about threadpool

[nbsdx]

No, the original use for this needed a static size at compile time, and I don't really want to split the code bases. I might add another file ThreadPoolDynanic.h with a dynamic version if I get around to it. But again, that would involve code duplication, which I'm against.

Translating it to be dynamically sized shouldn't be difficult, just remove the Size template argument, replace the std::array with a std::vector, and set the vector length in the constructor.