Convert Lightcurve to a subclass of torch.nn.Module?

[pscicluna]

Right now Lightcurve is a regular class. I think if it were converted to a Module, we could leverage more of torch's useful features and simplify the structure of the class a bit.

[pscicluna]

This looks like a good idea to make sure that GPU computing works more smoothly. both Lightcurve and Transformer need to be converted, and all their tensor attributes should be created with self.register_buffer(...)