Implemented a Base image dataset class

[dalloliogm]

Hi, I was working on the VAE bounty. That bounty is based on the COVID-19 dataset.

While looking at it, I wondered how users could customize the dataset, to use other images than this specific dataset.

So, I took the liberty of creating a BaseImageDataset class, and use that as based for the COVID19 dataset class.

I've also converted examples/covid19cxr_conformal.py into a notebook, as I thought that would be easier to read and fit better in an examples/ folder.

There are also a couple of test files for the new dataset class.

I've also improved the error message when the data is not found, suggesting users where they can download it.

I've added openpyxl as dependency because it is needed to read the metadata files.