Bug with rem_vertex! with next-to-edge atoms

[pw0908]

Hi!
Thank you so much for building this package! It's helped a lot with our ability to process molecular structures.

I just wanted to report an issue I've noticed with rem_vertex! when you remove an atom next to the final atom of a molecule. For example, in hexanol, if you try to remove the carbon next to the oxygen:

mol = smilestomol("CCCCCCO")
rem_vertex!(mol, 6) # KeyError: key Edge 6 => 7 not found

I believe that this is due to how the final vertex is treated in MolecularGraph:

MolecularGraph.jl/src/model/molgraph.jl

Lines 191 to 192 in fb4bb81

	nv_ == v || begin mol.vprops[v] = mol.vprops[nv_] end
	delete!(mol.vprops, nv_)

If this re-indexing is done, then this edge will no longer exist.

I'm not quite sure what the fix will be....

[mojaie]

Thank you for catching. I will merge your PR soon.