Add Seiffert spiral and shell trajectories

[Daval-G]

This PR adds two trajectories, with corresponding documentation & examples:

• Seiffert spirals, also known as Yarnball: quite recent analytical trajectories that rely on Jacobi elliptic functions and radially modulated Seiffert spirals to keep the gradient norm constant at a given radius.
• Seiffert shells, an exclusive trajectory using more conventional Seiffert spirals, i.e. defined over sphere surfaces and providing curves with constant speed and angular velocity. This trajectory is structured into concentric sphere similarly to helical and annular shells added previously.