Inspired by ANTsX/ANTsPy#437

Notice from the exception thrown there:

file 0000018600E3C1C0 does not exist . 
Exception Object caught: 

itk::ExceptionObject (0000000A275EBBC0)
Location: "unknown" 
File: D:\a\ANTsPy\ANTsPy\itksource\Modules\Registration\Common\include\itkCenteredTransformInitializer.hxx
Line: 31
Description: ITK ERROR: CenteredTransformInitializer(000001867F47D310): Fixed Image has not been set

On Windows the address is "0000018600E3C1C0" not "0x000018600E3C1C0", so it won't match the "0x" pattern, and ANTs attempts to read as a file from disk.

I've tried to modify the logic so that a hex string of the appropriate length (17 chars here) gets treated as a pointer also. My hope is that this will allow ANTsPy to work on Windows, but I can't test that directly.

The modified code does pass ANTs tests.