ENH: add components decorator

[ncullen93]

There are many functions that do not natively support vector images in the C++ but can work in the Python code by splitting the image, applying the function to each component, and then merging the image back. Such functions look like this:

@image_method
def crop_image(image):
   if image.has_components:
      return ants.merge_channels([crop_image(img) for img in ants.split_channels(image)])
   libfn = get_lib_fn('cropImage')
   return ants.from_pointer(libfn(image.pointer))

This PR removes that first if-statement by adding a decorator called @components_method. This decorator will let you add such split-apply-merge support to any function. It saves code and is more robust, so the function will now look like this:

@image_method
@components_method
def crop_image(image):
   libfn = get_lib_fn('cropImage')
   return ants.from_pointer(libfn(image.pointer))

And now the function knows to do the split-apply-merge thing if the image has components.

NOTE that this should not be applied to functions that natively support component images in the C++ code (although it wont really hurt I think), but I think that is very few of them.

[coveralls]

coverage: 84.728% (+0.04%) from 84.688%
when pulling 95f46ad on components-decorator
into c4f0e32 on master.