Bounty: VAE

[dalloliogm]

This is the PR for the "VAE" bounty.

Summary of changes:

• some quality of life improvements for COVID19CXRDataset, checking that data exists and expanding the path (e.g. ~/Downloads -> /home/user/Download)
• Added openpyxl requirements as it is needed to read the COVID-19 metadata file, which is Excel
• Implemented options for VAE, including image and timeseries
• Created a new notebook examples/chestXray_image_generation_VAE.ipynb to show example of using image-based VAE. Replaced a previous script that did not work
• Improved notebook on time series VAE as well examples/timeseries_mimic4.ipynb
• Converted examples/covid19cxr_conformal.ipynb to a notebook and fixed data loading code in examples/ChestXray-image-generation-GAN.ipynb (these are outside of the bounty, but they were broken and the code was similar enough to the VAE examples, so I fixed it)

[dalloliogm]

Should I convert the VAE class to Pytorch Lightining? I understood other classes in PyHealth are based on that, and I like PL syntax better.