Add support for running PennAI on Raspberry Pi

[JDRomano2]

7e11806 finishes adding the ability to run the PennAI docker containers entirely on the Raspberry Pi 400 (or another 4+ Gb model of the Raspberry Pi 4).

There is a new docker-compose-raspberrypi.yml configuration file that references separate Dockerfiles, effectively keeping the configuration isolated.

A major component of getting this working was building all of the Python dependencies as standalone wheel files that could be packaged into their own repository - https://github.com/EpistasisLab/pennai-arm64-deps. This is now included as a Git submodule in the docker/ directory. Users can populate the submodule by running:

$ git submodule init
$ git submodule update

Otherwise, the files will not be included in the main PennAI repository (useful for keeping the size of the repo to a manageable size).

More extensive testing and evaluation will be performed shortly.

[hjwilli]

Great start! There are a few steps before we merge into the main branch (making sure we can get the recommenders running on the pi, setting up ARM64 versions test runners, adding the ARM64 tests to the github CI if possible, updating the user and dev docs)