Add safety check for number of structures needed in advance of running the workflow

[JaGeo]

Can we add a check that we have enough structures for the split? I sometimes run into the issue of having too few structures for the fit.

[QuantumChemist]

I could add it to one of my open PRs

[JaGeo]

Or in a new one based on one of the open PRs to make review easier and avoid merge conflicts?