Release the GIL in ITK operations

[mrocklin]

First, a question: Does the ITK Python library release the GIL when it calls down to lower level C/C++ code?

When I run ITK operations within Dask I notice that my servers lock up. The most common cause of this is that some low level code is holding onto the GIL and not letting go. Is it possible that ITK does this?

If ITK does not release the GIL, could it?

xref https://discourse.itk.org/t/parallelize-itk-imageseriesreader-itk-4-12/1745/15

[thewtex]

@mrocklin thanks for the report. We will dive deeper into this.

:xref: #1065 , which was not sufficient.

[blowekamp]

This is how it is done in SimpleITK:
SimpleITK/SimpleITK@e9ce728