DataModule class for 4D-STEM

We need a simple file format with a PyTorch Lightning DataModule. There are examples how to do this in my littlebigimage repository and we can adapt those. The simplest file format is simply a directory holding one .npy file per beam location, and a single csv with nominal beam locations linked to filenames. The csv can hold other stuff like time, bias potential, tile angles, etc.

DataModules can be tough to test automatically but we could consider shipping an example small dataset and loading it as a test.