ITK 5 threading updates

[thewtex]

@N-Dekker @mstaring @stefanklein there was more to do than expected for the Windows Python packages, but we are green now :-).

I updated the sampler parallelism for ITK 5. This uses the dynamic threading methods -- when the TBB backend is enabled, the number of work units adjust dynamically at runtime based on the data and algorithm characteristics and CPU load. I did not have time to continue this throughout the rest of the codebase, but it serves as an example.