Add xsimlab process for steady-state moutain initial topography

[benbovy]

This is a common initial setup.

It may be possible to encapsulate a model run to dynamically create a steady state topography, maybe with some (configurable) resolution downgrade for speed-up. It might also be possible to leverage the dimensionless uplift/stream-power/diffusion relationship to adapt to the grid scale and to set a height scale.