sample_consensus: reduce SampleConsensusModelEllipse3D template instantiations

Move the Levenberg-Marquardt optimization logic into a non-templated
pcl::internal::optimizeModelCoefficientsEllipse3D function in the
source file, reducing redundant template instantiations from 61 to 1.
This results in a ~24% reduction in rebuild time for the
pcl_sample_consensus library.

Move the get_ellipse_point, dvec2ellipse, and golden_section_search
math helpers into pcl::internal in the header to allow shared access
without exposing them as class members.

Additionally:
- Fix projectPoints to write to projected_points[inlier] instead of
  projected_points[i++] when copy_data_fields is true, consistent with
  other SAC models
- Remove redundant headers (Eigen/Eigenvalues, sac_model_ellipse3d.h)
  from the .cpp, and pcl_exports.h from the .h (all transitively included)
- Restore explanation comments stripped during range-based for conversion
- Fix modernize-return-braced-init-list lint warning
- Match parameter signature with other optimized internal models
- Correct template instantiations for core and non-core point types

Author: AlrIsmail