Add more tests for radiation source model: radiation_tools

[kj5248]

From issue #3212. Add more tests for radiation_tools.py. Current breakdown of functions within file that are tested:

• [x] upstream_temperature (tested in test_radiation_profile)
• [x] target_temperature (tested in test_radiation_profile)
• [ ] specific_point_temperature
• [x] electron_density_and_temperature_sol_decay (tested in test_radiation_profile)
• [x] gaussian_decay
• [x] exponential_decay
• [x] ion_front_distance (tested in test_radiation_profile)
• [x] calculate_z_species
• [x] radiative_loss_function_values
• [ ] radiative_loss_function_plot
• [ ] calculate_line_radiation_loss
• [ ] linear_interpolator
• [ ] interpolated_field_values
• [ ] grid_interpolator
• [ ] pfr_filter
• [ ] filtering_in_or_out
• [ ] get_impurity_data
• [ ] detect_radiation
• [ ] make_wall_detectors
• [ ] plot_radiation_loads
• [ ] FirstWallRadiationSolver

[je-cook]

Not all of these will need full tests as some of their underlying functionality should already be tested elsewhere but it would be good to check