Added functionality for adding additional plots

[Zetison]

This PR enables the user to add additional plots from DataFrames or CSV-files.

[Copilot]

Pull request overview

This PR adds support for user-provided “additional plots” in the GUI, allowing plot series to be sourced from either in-memory DataFrames or CSV files and integrated into the “Available data” menu for plotting/normalization.

Changes:

• Adds additional_plots keyword to GUI(...), with validation and release-note documentation.
• Extends available-data initialization to ingest additional plot data (from DataFrame/CSV) and expose numeric columns as selectable plot containers.
• Adds test coverage to load/compare additional plots from both DataFrame and CSV inputs (and updates test dependencies/imports accordingly).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/setup_GUI.jl	Adds additional_plots kwarg, validates inputs, and stores config in gui.vars.
src/utils_GUI/GUI_utils.jl	Implements ingestion of additional plot data, adds CSV reading helper, extends selection API, and refactors time-period dict logic.
src/utils_GUI/results_axis_utils.jl	Improves time-axis type detection for GlobalDataContainer backed by a DataFrame.
test/test_results_IO.jl	Adds an integration test validating additional plot data from both DataFrame and CSV sources.
test/runtests.jl	Imports CSV/DataFrames for the new tests.
test/Project.toml	Adds CSV/DataFrames as test dependencies.
NEWS.md	Notes the new “additional plots” functionality.

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