Fix energy error plots

Show the change in energy (ΔE), not the energy.

Author: SebastianM-C

tutorials/models/01-classical_physics.jmd

@@ -350,10 +350,10 @@ energy = map(x->E(x...), sol.u)
 @show ΔE = energy[1]-energy[end]
 
 #Plot
-plot(sol.t, energy, title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
+plot(sol.t, energy .- energy[1], title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
 ```
 
-### Symplectic Integration
+##### Symplectic Integration
 
 To prevent energy drift, we can instead use a symplectic integrator. We can directly define and solve the `SecondOrderODEProblem`:
 
@@ -390,17 +390,17 @@ energy = map(x->E(x[3], x[4], x[1], x[2]), sol2.u)
 @show ΔE = energy[1]-energy[end]
 
 #Plot
-plot(sol2.t, energy, title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
+plot(sol2.t, energy .- energy[1], title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
 ```
 
-It's so close to zero it breaks GR! And let's try to use a Runge-Kutta-Nyström solver to solve this. Note that Runge-Kutta-Nyström isn't symplectic.
+And let's try to use a Runge-Kutta-Nyström solver to solve this. Note that Runge-Kutta-Nyström isn't symplectic.
 
 ```julia
 sol3 = solve(prob, DPRKN6());
 energy = map(x->E(x[3], x[4], x[1], x[2]), sol3.u)
 @show ΔE = energy[1]-energy[end]
 gr()
-plot(sol3.t, energy, title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
+plot(sol3.t, energy .- energy[1], title = "Change in Energy over Time", xaxis = "Time in iterations", yaxis = "Change in Energy")
 ```
 
 Note that we are using the `DPRKN6` sovler at `reltol=1e-3` (the default), yet it has a smaller energy variation than `Vern9` at `abs_tol=1e-16, rel_tol=1e-16`. Therefore, using specialized solvers to solve its particular problem is very efficient.