Make the repository pip install-able

[anand-bala]

This PR makes the repository installable as:

$ pip install git+https://github.com/StanfordASL/stlcg.git@<commit/branch>

or as

$ pip install https://github.com/StanfordASL/stlcg/archive/<commit/branch>.tar.gz

It also adds a pyproject.toml file for project metadata and configuration.

While the current version of the repository is also technically pip install-able, it does so by installing a top-level module named src under which you will find the modules src.stlcg, src.stlviz, src.utils, and src.logger.
This isn't really correct, nor is it ergonomic.

The other option is to copy just the stlcg.py file into your local, but that defeats the purpose of a package manager like pip. 😄

And thanks for your work!