Support lists with labels in coarse_graining.py

[jennyfothergill]

TODO comments from mbuild/coarse_graining.py

179-def _create_proxy_labels(real_thing, memo):
180-    if not is_leaf(real_thing):
181-        for label, part in real_thing.labels.items():
182-            if isinstance(part, list):
183:                # TODO support lists with labels
184-                continue

related to #872

[CalCraven]

Does anybody have a specific use case for this in mind? We may have a workaround up to this point since this was never addressed.