Change `vert_diff` config option

[haakon-e]

In default_config.yml, we have:

vert_diff:
  help: "Vertical diffusion [`false` (default), `VerticalDiffusion`, `true` (defaults to `VerticalDiffusion`), `DecayWithHeightDiffusion`]"
  value: "false"

This is a rather confusing combination of booleans and string flags. Additionally, many (12) configs have set this flag to:

vert_diff: true

which leads to the warning:

┌ Warning: Failed to convert `config_dict["vert_diff"] = true` to default type String, keeping original value
└ @ ClimaAtmos ~/ClimaAtmos.jl/src/solver/yaml_helper.jl:78

Ultimately, the expected vertical diffusion option is applied, but it is indeed rather confusing and not user-friendly.

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I think we should update the option as follows:

vert_diff:
  help: "Vertical diffusion [`~` (no diffusion, default), `VerticalDiffusion`, `DecayWithHeightDiffusion`]"
  value: ~

that is, removing true/false options.

---

If changing the valid options is a breaking change, I think we should consider changing the name to something more descriptive, e.g. vertical_diffusion_model.

---

This issue may be less relevant when the new simulation interface is merged.

[trontrytel]

Yeah! I'm planning to clean up horizontal/vertical diffusion options. Hopefully this year