[Spectral] Set default integrator to spectral

This reduces code duplication and complexity by removing all the cosseroots.*.integral packages and moving all cosseroots.*.spectral packages up one level. Since spectral integration is much faster than their integral counterparts, we should, in favor of speed and performance, remove the integral package from its public interface.

For the sake of completeness and verification of correctness of implementation, we may still keep the **.integral packages, but no longer allow selecting it through the property KinematicSolver but by explicit calling by the user.