ENH: Upgrade elastix to 2023-09-18, converts external initial transforms

Supports passing a transform retrieved by `GetCombinationTransform()` or
`GetNthTransform(n)` to `ConvertToItkTransform`, when an _external_ initial
transform was specified before running the registration.

Also including various performance improvements.

Including:

  pull request SuperElastix/elastix#946
  commit SuperElastix/elastix@7bcb2b3
  ENH: Write external initial transforms to ITK transform file

  pull request SuperElastix/elastix#949
  commit SuperElastix/elastix@601f465
  ENH: `ConvertToItkTransform` should support external initial transform

  pull request SuperElastix/elastix#950
  commit SuperElastix/elastix@7e883bf
  ENH: Add GetInitialTransform() and GetExternalInitialTransform()

  pull request SuperElastix/elastix#959
  commit SuperElastix/elastix@3979978
  PERF: Speed up ComputeJacobianTerms access to `vnl_sparse_matrix`

  pull request SuperElastix/elastix#960
  commit SuperElastix/elastix@4d028f7
  PERF: Speed up full, grid, sparse mask samplers by a local sampleVector

CMakeLists.txt

@@ -56,7 +56,7 @@ if(SKBUILD)
 endif()
 
 set(elastix_GIT_REPOSITORY "https://github.com/SuperElastix/elastix.git")
-set(elastix_GIT_TAG "23937288318b4ff42b225fbbfbc4abb128e06384")
+set(elastix_GIT_TAG "798ecaebab9d264d0f3cb375a1bf74174c41343c")
 FetchContent_Declare(
   elx
   GIT_REPOSITORY ${elastix_GIT_REPOSITORY}