Drop `ChemmineOB` as dependency and call OpenBabel more directly

[Aariq]

If I remember correctly, we only use ChemmineOB (via ChemmineR) in a few places:

• smiles2sdf()
• propOB()
• exactMassOB()
• smartsSearchOB()

It may be possible to write our own functions that use the Open Babel C++ library rather than requiring ChemmineOB. This would give greater control over installation and build issues, but would require more knowledge of including C++ code in R packages than I currently have.

[Aariq]

Rather than bundling OpenBabel with volcalc we could simply require users have it installed and make calls to it with system2() or the processx package. We could include a helper function to install OpenBabel possibly like hugodown does with the hugo CLI (https://github.com/r-lib/hugodown/blob/main/R/hugo-install.R)