Fix __repr__ method in abstract_site.Molecule and abstract_site.Residue

[chrisjonesBSU]

When poking around some things while working on #850 I got a strange error when trying to look at a Molecule instance in a notebook. There is an error in the __repr__ method of both the Molecule and Residue classes in abstract_site.py where it tries to call an attribute that doesn't exist.

This PR fixes that, and adds some extremely simple tests that should have caught this.

Also, I added fields for Residue and Molecule to the __repr__ method for Site as I think it provides helpful info.

>>> from gmso.abc.abstract_site import Molecule
>>> mol = Molecule()
>>> mol

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/chris/mambaforge/envs/flowermd-dev/lib/python3.11/site-packages/gmso/abc/abstract_site.py", line 27, in __repr__
    f"Molecule(name={self.name}, residue={self.residue}, isrigid={self.isrigid}"
                                          ^^^^^^^^^^^^
  File "/home/chris/mambaforge/envs/flowermd-dev/lib/python3.11/site-packages/pydantic/main.py", line 828, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
AttributeError: 'Molecule' object has no attribute 'residue'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.07%. Comparing base (5a4f17d) to head (c8c62ae).
Report is 29 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #851   +/-   ##
=======================================
  Coverage   94.07%   94.07%           
=======================================
  Files          65       65           
  Lines        6953     6953           
=======================================
  Hits         6541     6541           
  Misses        412      412           

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[chrisjonesBSU]

I'm not sure why all of these merge with upstream are showing up as commits here..I'll try and fix that after these recent PRs are done.

[CalCraven]

LGTM!