Repo reorg feb22

[sybenzvi]

PR to address #50. The following changes were made:

• The separate VoidFinder and Vsquared packages were copied into python/vast, making them submodules of a single vast package.
• A single top-level setuptools script was created based on the separate scripts previously available. The script was considerably simplified.
• The unit tests were merged into one folder and rewritten to use the Python unittest framework.
• The CircleCI test scripts were removed in favor of GitHub actions.
• The readthedocs YAML file was updated to account for the new directory structures (but document generation was not tested).

[sybenzvi]

@kadglass, I added the missing VF truth files needed by the unit test. The branch should work now, but I'll try to merge in @dveyrat's unit tests quickly so let's delay the merger by a few minutes.