Add quaternary metal dataset to Moleculnet

This is part of Phase 1 of infrastructure development for ANN potentials discussed in issue #2170 . Essentially, I am looking to add some robust materials datasets that can be used for ANN potential construction.

I would like to combine the validation and training data for LMNTO from the Cooper et al paper and add it as a MolNet dataset.

The datasets in question are released to the public and available here: https://archive.materialscloud.org/record/2020.0037/v1 .

The Materials Cloud archive contain lots of material data. This is just a possible starting point. My recent paper on ANN potentials actually used a TiO2 dataset provided by the Aenet team which may also be useful for our purposes.

Please let me know what you think!