Suggestion to make RDkit dependency optional

Hi Wout,

I recently added spectrum_utils plotting to MS²PIP (works great, thanks!). When running tests in GitHub Actions, I ran into the issue that RDKit is not installed using pip. Since RDKit is only used for molecule drawing, would you agree to make RDKit an optional dependency?

I would propose to put the rdkit imports in a try/except statement, and output a warning if the import fails. E.g. replace

spectrum_utils/spectrum_utils/plot.py

Lines 9 to 10 in 842de8b

    
           from rdkit import Chem 
        
           from rdkit.Chem import Draw

with

try:
    from rdkit import Chem
    from rdkit.Chem import Draw
except ImportError:
    warnings.warn("RDKit could not be imported. Some spectrum_utils functionality related to molecule drawing might lead to a NameError.", category=ImportWarning)

If you agree, I can open a PR.

I agree, I've already been thinking about doing that. Usage of RDKit within spectrum_utils is minimal and it's a heavy dependency (not really possible to install it without conda). I'll try to find some time this week after the PSI sessions to push a new version.

RDKit has been removed as a dependency. You can get spectrum_utils v0.3.5 from PyPI now, and from bioconda hopefully asap.

Thanks!