dependencies, PyPI and all that

[KrisThielemans]

It'd be nice to clean this up a bit more for easier install. Some suggestions:

• add requirements.txt with only petsird==0.2 and trimesh
• modify environment.yml to depend on trimesh and petsird-via-pip
• remove PETSIRD submodule
• move main.py to something like petsird_to_mesh.py (probably in main folder, not `python/'), and rename the project as well.

More advanced stuff:

• Not sure of the .devcontainer is useful for a Python only project
• We could even put this on PyPI via pyproject.toml and GitHub actions (as per https://github.com/ETSInitiative/PETSIRD/blob/main/.github/workflows/ci.yml and https://github.com/ETSInitiative/PETSIRD/blob/main/python/pyproject.toml)

[KrisThielemans]

Most of this has been addressed in #15 . Remaining items:

• move main.py
• put on PyPI