Add a unit grid generator

Main changes of this PR

This PR adds a unit grid generator with adjustable x, y, z dimensions. Defaults to 2D by default (z=1).

[ ] I used the pre-commit hook and used pre-commit run --all to apply all available hooks.
[ ] I added a test to cover the proposed changes in our test suite.
[ ] I updated the documentation in docs/README.md.
[ ] I updated potential breaking changes in the tutorial precice/tutorials/aste-turbine.