Field trees

[milankl]

One can now do

julia> tree(simulation)
Simulation{PrimitiveWetModel}
├┐prognostic_variables
│├ trunc
│├ nlat_half
│├ nlev
│├ n_steps
│├┐layers
││└┐timesteps
││ ├ trunc
││ ├ vor
││ ├ div
││ ├ temp
││ └ humid
│├┐surface
││└┐timesteps
││ ├ trunc
││ └ pres
│├┐ocean
││├ nlat_half
││├ time
││├ sea_surface_temperature
││└ sea_ice_concentration
│├┐land
││├ nlat_half
││├ time
││├ land_surface_temperature
...

to get a full field tree (a tree of all fields and their fields as long as they are defined in SpeedyWeather) printed. It's convenient to visualise nested structs. Also added to the docs to have that automatically documented.

@natgeo-wong this is a first step towards "a more detailed explanation of modules" by which I think you mean the components of a SpeedyWeather simulation (avoiding the term "module" because of its use in Julia)