Fix ligand from graphmol bug

[apayne97]

Description

While Ligand.from_oemol requires an OEMol input, the code mostly still works if provided an OEGraphMol object. Some API calls don't work, however - in particular NumConfs, as OEGraphMol objects can only have 1 conformer (whereas OEMol objects can have multiple). To preserve the ability to run Ligand.from_oemol with an OEGraphMol object, I've added a small test for this and made a small code to make sure that the .NumConfs method is only run with an OEMol object.

Status

• Ready to go

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.99%. Comparing base (643e041) to head (63eff92).

❗ There is a different number of reports uploaded between BASE (643e041) and HEAD (63eff92). Click for more details.

HEAD has 18 uploads less than BASE

Flag	BASE (643e041)	HEAD (63eff92)
unittests	19	1

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