The Tolerances trait: the a_tol, a_tol_full and r_tol setters move off the
inherent impls onto a trait, with absolute_tolerance and relative_tolerance
getters, so code generic over the integrator can set and read them.
- repository pointed at the old host and the old repo name; this is the link
crates.io and docs.rs display
- the readme example imported `differential_equations`, so it could not
compile under the crate's published name
- `cargo bench` did not build: `Solution` borrows the ODE, so `.solve()`
cannot be called on a temporary `Problem`. Bind it first.
Bumps the version to 0.3.0 and keeps the roadmap notes out of the published
artifact. No source changes: this tags the BS3, Vern7, Rosenbrock23 and
PIDController work that has accumulated on main since 0.2.3.
connor
