large-scale condensation at <100%

[minqi6]

The problem is that large-scale condensation at <100% currently gives negative precipitation as the ImplicitCondensation isn't correctly generalised for other thresholds than 1=100%

from #614

using SpeedyWeather
spectral_grid = SpectralGrid(trunc=31, nlayers=8)
large_scale_condensation = ImplicitCondensation(spectral_grid, relative_humidity_threshold = 0.8)
convection = SimplifiedBettsMiller(spectral_grid)
model = PrimitiveWetModel(; spectral_grid, large_scale_condensation, convection)
simulation = initialize!(model, time=DateTime(2007, 1, 1))
run!(simulation, period=Day(20), output=true)

[milankl]

Yep, fantastic!

[milankl]

The tests are running now, would also want to write some additional ones simply testing what you tried with 0.8, 0.9, 1.0 and checking that a 5-day simulation is stable?

[milankl]

1. We have a changelog that needs to be updated with every pull request, you can write something like - bugfix: large-scale condensation also at <100% [#609](https://github.com/SpeedyWeather/SpeedyWeather.jl/pull/609) this just helps us to keep track of changes that were made in different versions of the model. Past versions have this too but it sometimes also good to have one file with a list of all changes listed by version

2. The tests we'd need would look something like

@testset "Large-scale condensation" begin
    spectral_grid = SpectralGrid(trunc=31, nlayers=8)
    @testset for r in (0.8, 0.9, 1.0)
        large_scale_condensation = ImplicitCondensation(spectral_grid, relative_humidity_threshold=r)
        model = PrimitiveWetModel(spectral_grid; large_scale_condensation)
        simulation = initialize!(model)
        run!(simulation, period=Day(5))
        
        precip = simulation.diagnostic_variables.physics.precip_large_scale

        for ij in eachindex(precip)
            @test precip[ij] >= 0   # precipitation should always be non-negative
        end
    end
end

I would create a new file in test/large_scale_condensation.jl which then needs to be included in test/runtests.jl