Reformat loaders for different smiles paths

[CalCraven]

PR Summary:

Updating the loading paths to deal with issues loading long strings as smiles. Initial loading uses the Path package to check if the path exists, before checking to see if rdkit can load a valid smiles file. However, Path throws an error if the Path is too long:

"name": "OSError",
"message": "[Errno 63] File name too long: 'CC1C(=O)NC(C(=O)NC(CSC(C2=C(C3=CC4=C(C(SCC(C(=O)N1)NC(=O)C(CCCCN)NC(=O)C(CCC(=O)N)NC(=O)C(C(C)C)N)C)C(=C([N-]4)C=C5C(=C(C(=N5)C=C6C(=C(C(=CC2=N3)[N-]6)C)CCC(=O)[O-])CCC(=O)[O-])C)C)C)C)C(=O)NC(CC7=CNC=N7)C(=O)NC(C(C)O)C(=O)NC(C(C)C)C(=O)NC(CCC(=O)[O-])C(=O)[O-])CCC(=O)N.[Na+].[Na+].[Na+].[Na+].[Fe+2]'",

This PR better handles this type of smiles, and also splits the handling of the rdkit and pybel backends for smiles loading, both from files and from strings. Discussion with @mattwthompson helped delineate the more specific loaders for these different methods.

PR Checklist

---

• Includes appropriate unit test(s)
• Appropriate docstring(s) are added/updated
• Code is (approximately) PEP8 compliant
• Issue(s) raised/addressed?
• Handle lists of smiles
• Handle many smiles in a .txt file
• Handle many smiles in a .smi file

[codecov]

Codecov Report

Attention: Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.89%. Comparing base (a4c41c5) to head (d7ea5e2).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
mbuild/conversion.py	95.12%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1211      +/-   ##
==========================================
+ Coverage   85.73%   85.89%   +0.15%     
==========================================
  Files          53       53              
  Lines        4802     4799       -3     
==========================================
+ Hits         4117     4122       +5     
+ Misses        685      677       -8     

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[chrisjonesBSU]

LGTM. Left a couple comments that should be quick changes. Thanks!