Redesigned AccessByItk macros and cleaned up header files.

• The AccessByItk macros use preprocessor magic now, instead of template magic. This is as flexible as the old style and as configurable as the previous type list approach.
  
  The macros are mostly backwards compatible, except for AccessFixedPixelTypeByItk and AccessFixedTypeByItk which take sequences of types and dimensions.
  
  Further, up to 25 arguments can now be passed to the access function with the same macro.

• The CastToItkImage function template declaration has been moved to mitkImageCast.h and corresponding header files and include directives have been cleaned up.

• Core classes have been reverted to use AccessByItk macros again (instead of the previous type list / functor approach).