Fixing python3 imports for UFO models

Hi authors,

I'd really like to see if we can improve upon the current find_ufo_path:

https://github.com/mg5amcnlo/mg5amcnlo/blob/main/models/import_ufo.py#L69

to make the importing of models a bit more pythonic. That'd be a nice step towards giving us a little more power over the organization of our models. For a typical python module, you can do an:

import my_module
my_module.__path__

to get back the path to the model in case that's needed for anything. That means we could do something like

try:
  import my_model
  return my_model.__path__

Of course, please let me know if there's something fundamental I'm missing here that would make this a very bad idea. Assuming it's not, that works nicely in python2, e.g.:

>>> import vector_LQ_UFO
>>> vector_LQ_UFO.__path__
['/cvmfs/atlas.cern.ch/repo/sw/Generators/madgraph/models/latest/vector_LQ_UFO']

In python3, relative imports are no longer allowed (see e.g. https://nerdparadise.com/programming/python/import2vs3). That means when doing this import of the model, I hit a python error:

>>> import vector_LQ_UFO.propagators
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/cvmfs/atlas.cern.ch/repo/sw/Generators/madgraph/models/latest/vector_LQ_UFO/__init__.py", line 1, in <module>
    import object_library
ModuleNotFoundError: No module named 'object_library'

because the __init__.py of the UFO models use a whole bunch of relative imports:

import object_library

import particles
import couplings
import lorentz

instead of absolute imports, like:

from . import object_library
from . import particles
from . import couplings
from . import lorentz

This model is otherwise "python3-ready", except for the relative imports that show up everywhere (and this seems generically true for all the UFO models we use, I'm not just picking on whoever wrote this particular model).

The question is: is there any issue with changing these to absolute imports and moving in a direction where this is a bit more pythonic? As I said, I'm happy to help implement some of these changes, and to have them protected behind e.g. try/except blocks so that they won't affect users who don't want to use them, but I think it'd be a nice thing to do.

Thanks,
Zach