Remove ASE requirement from build

[nedtaylor]

In setup.py, ASE is defined as a minimum build requirement. However, I do not believe this is true.

Test building without ASE and remove requirement if possible.

[nedtaylor]

install_requires in setup.py are not used for installation, these are used for running the code. I think it best to say that the code relies on ASE, just because it makes it easier to interact with. If people want to remove ASE, they can manually delete it from the setup and pyproject.

build-system.requires in pyproject.toml are build requirements.