Add ion capabilities

[lilyminium]

Fixes #138

Changes made in this Pull Request:

• This puts in necessary technical fixes to allow NAGL to work with ions. The PR needed to copy over code from DGL; we should revisit if combine_frames doesn't allow for no edges dmlc/dgl#7826 is fixed.
• It also adds tests for an "rc4", which is the rc3 model of NAGL1 manually modified to accept atoms with 0 bonds as a feature (weights are set to 0). The tests test that for real molecules we get the same charges out as rc3, but also adds / un-xfails some ion tests.

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 15.38462% with 44 lines in your changes missing coverage. Please review.

Project coverage is 68.71%. Comparing base (1d7e4ff) to head (8be0f3c).
Report is 1 commits behind head on main.

Additional details and impacted files

[lilyminium]

Attribution and licensing that part under Apache 2.0 should be sufficient I think? https://law.stackexchange.com/questions/59999/can-i-copy-pieces-of-apache-licensed-source-code-if-i-attribute