Improve help page of the CLI

[schneiderfelipe]

Currently, overreact -h is not that useful. We need

• Properly describe all options, including their units if applicable
• Tell about the program and guide the user through online documentation, citation, license, etc.

That last point can be set into a single description string and wired so that both the output header and the help page show the same preset information.

[schneiderfelipe]

Still work to do, not closing. This will wait a bit until I work on the docs.