Reorganize packaging and hopefully fix tests

[tsalo]

Closes #269.

Changes proposed:

• Remove the Docker and Singularity build/test steps from the CircleCI config.
• Replace setuptools-based packaging with hatch. This means dropping setup.py, setup.cfg, tox.ini, and requirements.txt in favor of pyproject.toml. It should also infer the correct version string, rather than relying on a hardcoded version in the code.
• Move the test files into cubids/tests.
• Add a CITATION.cff file with the Zenodo authors and the CuBIDS paper.
• Remove unused package*.json files.

[tsalo]

I'm stumped. I need to install CuBIDS in the CircleCI Docker image in order to access the package's version, because the version is then fed into the built CuBIDS Docker image, but I can't seem to get a base image with Docker installed within it and Python 3.8+ (which is necessary to install a Python library using pyproject.toml).

[tsalo]

CircleCI has both Docker executors and machine executors. The machine executor can run Docker without having to set up a remote docker.

[tsalo]

It's working!

[tsalo]

@mattcieslak if we can tackle this PR next, it should lead into #268 pretty cleanly.