ENH: Bump Python package version to 0.18

[thewtex]

Include initial transform improvements.

[N-Dekker]

Please note that the added ElastixRegistrationMethod::SetExternalInitialTransform is very new, so it might still have some flaws. Use with care 😃

[thewtex]

@N-Dekker yes, I make a note in the release notes that it is available for initial experimental testing. :-) Thanks!

[N-Dekker]

I'm preparing another extension to elastix, TransformixFilter::SetExternalTransform which would allow specifying any arbitrary ITK transform to be used by transformix. So it's basically complementary to ElastixRegistrationMethod::SetExternalInitialTransform Wonder if this should still be included with ITKElastix 0.18. Otherwise, it's also fine to me to have it in 0.19.

[dzenanz]

The ability to supply ITK transform and get it out of ElastixRegistrationMethod reduces the need for transformix. So I don't think there is any hurry for that functionality.

[N-Dekker]

The ability to supply ITK transform and get it out of ElastixRegistrationMethod

Honestly I did not yet add a GetExternalInitialTransform() member function to ElastixRegistrationMethod. Is any hurry for that one, GetExternalInitialTransform()? Obviously, it's pretty trivial to write (unlike SetExternalInitialTransform).

[dzenanz]

By "get ITK transform out of ElastixRegistrationMethod" I meant this:

comb_transform = elastix_object.GetCombinationTransform()
itk_transform = elastix_object.ConvertToItkTransform(comb_transform)

[tbirdso]

If there are no objections I will go ahead and merge this shortly.

[N-Dekker]

By "get ITK transform out of ElastixRegistrationMethod" I meant this:

comb_transform = elastix_object.GetCombinationTransform()
itk_transform = elastix_object.ConvertToItkTransform(comb_transform)

Sorry @dzenanz ConvertToItkTransform does not yet support converting a combination transform that contains an "external" ITK transform. To be continued...!