Use functions for saving flow solutions

[gnawin]

What and Why

The current implementation for saving flow solutions (see code below) in the save_solution_to_file function uses a block for DataFrames.flatten and DataFrames.transform.

    output_file = joinpath(output_folder, "flows.csv")
    output_table =
        DataFrames.select(dataframes[:flows], :from, :to, :rp, :timesteps_block => :timestep)
    output_table.value = solution.flow
    output_table = DataFrames.flatten(
        DataFrames.transform(
            output_table,
            [:timestep, :value] =>
                DataFrames.ByRow(
                    (timesteps_block, value) -> begin # transform each row using these two columns
                        n = length(timesteps_block)
                        (timesteps_block, Iterators.repeated(value, n)) # e.g., (3:5, [30, 30, 30])
                    end,
                ) => [:timestep, :value],
        ),
        [:timestep, :value], # flatten, e.g., [(3, 30), (4, 30), (5, 30)]
    )
    output_table |> CSV.write(output_file)

After #564, there is a similar function defined for storage (see code below).

function _interpolate_storage_level!(df, time_column)
    DataFrames.flatten(
        DataFrames.transform(
            df,
            [time_column, :value, :processed_value] =>
                DataFrames.ByRow(
                    (period, value, start_value) -> begin
                        n = length(period)
                        interpolated_values = range(start_value; stop = value, length = n + 1)
                        (period, value, interpolated_values[2:end])
                    end,
                ) => [time_column, :value, :processed_value],
        ),
        [time_column, :processed_value],
    )
end

Since this function are added for the storage, the first block code can be simplified further by also using this function. For that purpose, the function may need to be modified a bit.

Possible Drawbacks

It will make the code cleaner, but that may not save many codes (since some conditions would need to be added to the functions and currently there is only this part that needs to be refactored).

It may lose or improve readability.

Related Issues

No response

[datejada]

Maybe obsolete if we use dataframes for outputs #115

[datejada]

Also see comments on #688