VMEX 0.11.8

• vmex --trace converges without a step to choose. The default
  integrator is per-particle error-controlled Dopri8 at tolerance 3e-7
  (ESSOS 0.20 adaptive stepping); --trace-tolerance sets it and
  --trace-timestep becomes the first trial step. On twenty-one equilibria
  the worst energy error is 1.7e-4, against up to 0.27 for the earlier
  fixed RK4 step, at about 2.6 times its cost (#554).
• The default Boozer mode cut is 2e-4, safe on twenty equilibria, and every
  run reports its largest energy error and how to tighten it above 1e-3
  (#553). The trace guide gives the mode-cut, timestep, K and integrator
  studies on the corrected tracer (#552).
• The optimization Jacobian is cached persistently (#549). New examples:
  low-bootstrap QA, a QI drift study and a DKX-bootstrap QI (#550).
• Floor essos>=0.20.0.