create a more elaborate demo-example for the Lake package

In GitLab by @luitjansl on Jun 14, 2023, 17:39

idea for such an example:
create a domain with a topographical depression in the middle. create a lake in the depression. The head at the boundaries should be set above the bottom of the depression. Now the lake should slowly fill up. Then lower the head at the boundaries. Now the lake should slowly infiltrate and run dry, but is should keep the heads around it a bit higher than those at the boundary while doing so.

The issue names the Lake package but no files, tests, or demo entry point. Start by locating existing Lake examples and the package's example conventions, then define the domain, boundary heads, filling phase, and drying phase described here. Done should be a runnable example showing the lake fill, boundary lowering, infiltration, and maintained surrounding heads.