Pin MLJFlux at version 0.4

[trontrytel]

The NN aerosol activation emulator training was broken in the longruns because the API of MLJFlux package have changed:

ArgumentError: Flux.jl optimiser detected. Only optimisers from Optimisers.jl are supported.

I tried updating to using Adam optimiser from the Optimisers package as they want us to do. However I then get an error:

ERROR: Optimisers.jl cannot be used with Zygote.jl's implicit gradients, Params & Grads

At least for now I'm just pinning the MLJFlux package at the older version.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.01%. Comparing base (f73555a) to head (1973e64).
Report is 1 commits behind head on main.

Additional details and impacted files

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##             main     #417   +/-   ##
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  Coverage   97.01%   97.01%           
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  Files          37       37           
  Lines        1440     1440           
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  Hits         1397     1397           
  Misses         43       43           

Components	Coverage Δ
src	98.95% <ø> (ø)
ext	69.79% <ø> (ø)