Add QCArchive datastructures

[wiederm]

Description

We want to provide a dataset structure for the most common datasets that are provided through the QCArchive infrastructure. These datasets should play nicely with the pytorch dataloader objects and inherit from torch.utils.data.Dataset.

Todos

• [x] Outline general structure
• [x] Implement QM9 toy dataset
• [ ] Tests are passing

Questions

• [ ] How to best query different labels?
• [ ] Is there any use case for different datapoints beside coordinates?
• [ ] Is a view the best structure to perform queries on?

Status

• [x] Ready to go

[chrisiacovella]

I think, after a few minor changes, we should just merge this PR so we can create a separate one aimed at fixing the CI workflow.