What should be in the abel namespace?

[MikhailRyazanov]

Let's discuss the issue raised in #378. Namely, abel/__init__.py imports

from .transform import Transform
from .tools.center import center_image

so that users can call the short forms abel.Transform and abel.center_image, without specifying corresponding submodules.
I guess, everybody agrees that abel.Transform is desirable, as it gives the simplest unified access to basic functionality, but the opinions regarding abel.center_image were different. Moreover, the latter wasn't documented anywhere.

So the question is whether we should leave/recommend only abel.Transform and deprecate abel.center_image (to be removed completely later), or to document abel.center_image and recommend it for basic tasks. If we incline towards the second option, then it might be also a good idea to add

from .tools.circularize import circularize_image

because it's also quite useful (sometimes even necessary) in practice and, unlike center_image, is not available through Transform options.