add SimpleITK.Image to ANTsImage conversion

[dyollb]

• add helper function to get an ANTsImage from a SimpleITK.Image
• add test that does the roundtrip

[coveralls]

coverage: 85.061% (-0.3%) from 85.358%
when pulling 3433a63 on dyollb:sitk_to_ants
into f079ba6 on ANTsX:master.

[dyollb]

I guess the coverage went down because the test doesn't run unless SimpleITK is installed. Should I install it in the CI?

[cookpa]

This looks very useful. We have other functions like from_numpy() so maybe we should name similarly, eg from_sitk(sitk image) and to_sitk(ANTsImage)?

[dyollb]

This looks very useful. We have other functions like from_numpy() so maybe we should name similarly, eg from_sitk(sitk image) and to_sitk(ANTsImage)?

would you also rename the module?

Update: I renamed the functions.

[cookpa]

This is shaping up, @dyollb would you mind editing ants/utils/__init__.py to import the functions, so they can be called as ants.from_sitk etc

[cookpa]

This LGTM, thanks