STYLE: Remove AdvancedCombinationTransform member function pointers

[N-Dekker]

Removed those nine member function pointers as well as the 28 member functions that they could point to, and added nine switch (m_SelectedMethod) statements instead.

[N-Dekker]

To my surprise, the cleanup proposed by this PR appears to possibly slow down a "default" registration! 😞 At least, that's what I observed on VS2019 Release, on the following code:

constexpr auto imageDimension = 3U;
using PixelType = float;
using ImageType = itk::Image<PixelType, imageDimension>;

const auto fixedImage = itk::ReadImage<ImageType>("F:\\ITKElastix\\examples\\data\\CT_3D_lung_fixed.mha");
const auto movingImage = itk::ReadImage<ImageType>("F:\\ITKElastix\\examples\\data\\CT_3D_lung_moving.mha");

for (int i{}; i < 5; ++i)
{
  elx::DefaultConstruct<itk::ElastixRegistrationMethod<ImageType, ImageType>> registration{};

  // Do a registration, using the default parameter maps of ElastixRegistrationMethod. May take some time!
  registration.SetFixedImage(fixedImage);
  registration.SetMovingImage(movingImage);

  using namespace std::chrono;
  const auto timePointBeforeUpdate = high_resolution_clock::now();
  registration.Update();
  const auto timePointAfterUpdate = high_resolution_clock::now();
  const auto durationSeconds = duration_cast<duration<double>>(timePointAfterUpdate - timePointBeforeUpdate);
  std::cout << " Duration: " << durationSeconds.count() << " second(s)" << std::endl;
}

At the main branch: ~33 seconds. With this PR: ~37 sec.