Provide optional dependencies for opm

[MatthewFlamm]

The documentation suggests optionally installing opm python package using pip install opm

For not macOS users, to install the Python OPM package, execute in the terminal pip install opm.

It would be convenient for users to install everything in one go using pip install pyopmspe11[opm] by declaring it as an optional dependency in pyproject.toml. https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#dependencies-and-requirements.

While nice for users, this is not directly relevant to the review in openjournals/joss-reviews#7357.

[daavid00]

Thanks @MatthewFlamm for the suggestion, this has been added in #103