Improve user-overriden functions

[nefrathenrici]

Three current functions are provided by the user as hooks in the calibration lifecycle: forward_model, observation_map, analyze_iteration.

The information passed into these functions is quite limited, and pipelines commonly end up loading the EKP struct and accessing global variables to compensate. We should provide a data structure containing these to each function to ensure that they have the needed variables by default.

Rough sketch of the data structure:

struct CalibrationContext
    iteration::Int
    member::Int
    output_dir::String
    ensemble_size::Int
    prior::Union{ParameterDistribution, Nothing}
    ekp::Union{EKP.EnsembleKalmanProcess, Nothing}
end

New function calls will be:

• forward_model(ctx::CalibrationContext)
• observation_map(ctx::CalibrationContext)
• analyze_iteration(ctx::CalibrationContext, g_ensemble)