Add the Störmer-Verlet method + symplectic test changes

[packquickly]

Changes:

• Add the Störmer-Verlet method
• Add all_symplectic_solvers to test/helpers.
• Change test_semi_implicit_euler to a parameterised test test_symplectic_solvers which includes Störmer-Verlet
• Add an order test test_symplectic_ode_order similar to test_ode_order using a simple harmonic oscillator as the base problem instead of the linear equation in test_ode_order.

Störmer-Verlet is implemented in the general partitioned form updating p and q with generic vector fields f(p) and g(q) rather than the more common version which assumes g(q) = q. This is consistent with the Diffrax implementation of SemiImplicitEuler.