WIP: detect and handle errors from OpenBabel via propOB

[Aariq]

Attempt at solving #56 by using callr to run propOB in an isolated R session where I can capture stderr. The downside is that it appears to be quite a bit slower, but it does appear to work. Still much to do:

• [ ] Integrate into get_fx_groups()
  • [ ] Pass ob_err_pass from get_fx_groups() (actually, maybe this argument shouldn't be part of safe_propOB() but rather higher up and have the effect of choosing between ChemmineR::propOB() and safe_propOB(). That way can use faster option when there aren't errors.)
  • [ ] Make sure results have NAs in the correct places
  • [ ] Add (list) column with Open Babel messages?

[Aariq]

Decided it is likely better to check for the symptoms of invalid SDFs rather than to try to capture the errors. See discussion in #85. Will leave this open until we are sure that this alternative approach is better, then will close this PR.

[Aariq]

Closing this as it was superseded (mostly) by #104