ENH: Add SetDefaultNumberOfThreads

[thewtex]

Addresses: InsightSoftwareConsortium/ITK#113

[thewtex]

@mrocklin please take a look

[mrocklin]

From a Python user's perspective I would prefer an API like ...

import itk
itk.set_nthreads(4)

There are a few other libraries (blosc comes to mind) that do something similar.

However, with proper documentation what's here also works. I imagine that the sorts of people who are going to want to change the default number of threads are also going to have a decently high tolerance for writing lots of code.

Thank you all for working on this.

[jhlegarreta]

Although @mrocklin 's comment seems fair, I guess it is not trivial to implement that under ITK's framework. If you think it is feasible, an issue could be opened in the ITK repository and cross-reference this example.

In the meantime, I think merging the example would help users have a better grasp of the current multi-threading mechanism use.

[mrocklin]

Yeah, that was my guess as to what was happening. Thought I'd mention it anyway. Thanks all!

[dzenanz]

It should not be hard to implement. A stand-alone function itk::set_nthreads:

void 
set_nthreads(ThreadIdType numberOfThreads)
{
  itk::MultiThreaderBase::SetGlobalDefaultNumberOfThreads(numberOfThreads);
}

should be about enough.

[dzenanz]

While such a function wouldn't make much sense in C++, we could make it available only in Python. Thoughts?

[thewtex]

Thanks for the reviews!

The CI failures are unrelated, so merging.

If there is demand for it, we could add itk.set_nthreads by defining it in itkExtras.py.

It would look like:

def set_nthreads(number_of_threads):
    threader = itk.MultiThreaderBase.New()
    threader.SetGlobalDefaultNumberOfThreads(number_of_threads)