Add subseasonal calibration pipeline

[nefrathenrici]

Purpose

This PR adds a subseasonal calibration pipeline using ERA5 observations. The pipeline runs September months from 2018 to 2021 with the wxquest_diagedmf.yml config, collecting monthly surface flux diagnostics. The observations consist of ERA5 surface fluxes (latent heat, sensible heat, evaporation) with a latitude-weighted, diagonal scalar covariance structure. The only parameter currently calibrated is precipitation_timescale (tau_precip). The implementation uses a modular structure with separate files for observation generation, loss functions, model interface, and calibration orchestration. This pipeline will serve as a template for future coupled-model calibration.

Content

• Introduces CalibrateConfig struct (from @ph-kev) to manage calibration settings (minibatch size, iterations, date ranges, spinup/extend periods)
• Moves calibration code from root to experiments/calibration/subseasonal/ directory
• New files for the pipeline: generate_observations.jl, model_interface.jl, observation_map.jl, observation_utils.jl
• Moves the already existing pipeline to experiments/calibration/test

Sample longrun

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