To help extension developers transition their code base, a migration guide is also
available on the wiki.
See https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide/ITK4-to-ITK5

This commit implements the following changes

* Compatibility with refactor of threading models in ITK: Fix compilation
  using ProcessingThreader class instead of PlatformMultiThreader.
  ITKv5 reorganized the multi-threading capabilities of ITK into a more modular organization
  and naming to support several modern parallelism paradigms.

  Note that these changes allow Slicer to compile with ITKv5, but does not take advantage
  of any enhancement supporting new threading models. ProcessingThreader
  is still the old-renamed PlatformMultiThreader.

  These changes were originally associated with PR https://github.com/Slicer/Slicer/pull/1057

* Fix compilation issue related to ComputeJacobianWithRespectToPosition parameter mismatch.

...