Profile and fix import time

[lilyminium]

Expected behavior

We should aim for naive imports of openff.nagl and from openff.nagl import GNNModel to be as fast as possible. At the moment they most likely import heavy dependencies that may not all be necessary: #57

Actual behavior

Code to reproduce the behavior

import openff-nagl

...

Current environment

• Which version are you using? (run python -c "import openff-nagl; print(openff-nagl.__version__)")
• Which version of Python (python -V)?
• Which operating system?
• What is the output of pip list?
• If you use conda, what is the output of conda list?