XBeach parameter nesting #30

Certain parameters in XBeach are only valid with some specific types of choices. For example, nhq3d is only relevant if the nonh wavemodel is selected.

In rompy-xbeach, this is being handled by defining pydantic models for certain types that are associated with other specific parameters. Ultimately, a flat key-value params.txt file is rendered, but this allows controlling which parameters are available with which options.

The idea of this issue is to describe some of these definitions, and make sure the correct parameters are being prescribed to the correct pydantic models.