Improve interface to radas data

[tbody-cfs]

Updates atomic data handling to work with https://github.com/cfs-energy/radas/tree/v2025.6.1.

Main fix

By default, radas@v2025.6.1 computes only a single ne_tau value. This breaks a line of logic in the AtomicData class. Although this can be fixed with a small patch, I decided it was worth refactoring some of the code to use a neater CoeffInterpolator extending RectBivariateSpline to handle the log-space interpolation.

Related fixes

• radas_config.yaml added to cfspopcon to make it clearer how radas is being configured.
• calc_impurity_charge_state and calc_impurity_radiated_power_radas simplified and made unit-aware, using the .groupby(dimension).map(function) xarray functionality.
• The latest version of radas updates several packages, which led to a number of mypy changes.
• Changed ne_tau for the SPARC PRD to 0.5 * ms * n20 from 1 * ms * n20, matching the value evaluated by radas
• Updates test_atomic_data for new implementation

Drive-by fixes

• poetry was updated to version 2.1.3 (locally and in Github actions)
• get_item and get_values helper methods used to replace common patterns
• type hints for evaluate_density_and_temperature_profile_fits simplified
• Fixed an error where wraps_ufunc error handling would fail if function is unnamed.
• Updates setuptools (closes build(deps-dev): bump setuptools from 71.1.0 to 78.1.1 #124)

[MishaVeldhoen]

Looks good! Left a couple of small comments, mostly nit.