Fix double tap tangential hit bug, update variable names.

[cgyurgyik]

Description

• Fix double tap in tangential hit. #167
• Overhaul and clean up of the code. Moves FP comparison functions in a separate *_util.h file.
• Replaces radialHitData with radialHitMetadata, as well as another few variable name changes.
• Use a singular HitParameter struct.

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

    TEST(SphericalCoordinateTraversal, TangentialHitNoDoubleIntersectionWithSameVoxel) {
        const BoundVec3 sphere_center(0.0, 0.0, 0.0);
        const double sphere_max_radius = 10.0;
        const std::size_t num_radial_sections = 4;
        const std::size_t num_polar_sections = 1;
        const std::size_t num_azimuthal_sections = 1;
        const svr::SphereBound max_bound = {.radial=sphere_max_radius, .polar=TAU, .azimuthal=TAU};
        const svr::SphericalVoxelGrid grid(MIN_BOUND, max_bound, num_radial_sections, num_polar_sections,
                                           num_azimuthal_sections, sphere_center);
        const BoundVec3 ray_origin(-2.5, 0.0, 10.0);
        const FreeVec3 ray_direction(0.0, 0.0, -1.0);
        const Ray ray(ray_origin, ray_direction);
        const double t_begin = 0.0;
        const double t_end = 30.0;
        const auto actual_voxels = walkSphericalVolume(ray, grid, t_begin, t_end);
        const std::vector<int> expected_radial_voxels = {1, 2, 3, 2, 1};
        const std::vector<int> expected_theta_voxels = {0, 0, 0, 0, 0};
        const std::vector<int> expected_phi_voxels = {0, 0, 0, 0, 0};
        expectEqualVoxels(actual_voxels, expected_radial_voxels, expected_theta_voxels, expected_phi_voxels);
    }

Similar test for Cython code.

Checklist:

• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes