Hello!

Using Beeler-Reuter model from the README I can list **observables**:

```
julia> ml = CellModel("models/beeler_reuter_1977.cellml.xml")
julia> observed(ml.sys)
39-element Vector{Equation}:
slow_inward_current_f_gate₊V(time) ~ membrane₊V(time)
slow_inward_current₊f(time) ~ slow_inward_current_f_gate₊f(time)
⋮
time_dependent_outward_current₊i_x1(time) ~ membrane₊i_x1(time)
slow_inward_current₊i_s(time) ~ membrane₊i_s(time)
julia> ml.sys.membrane₊i_Na
var"##ODESystem#257₊membrane₊i_Na"(time)
```

But I do not understand how to get them calculated over time (and then plot, for example) after I solve the model.

For example, I would like to do this like:

```
prob = ODEProblem(ml, (0, 42.0))
sol = solve(prob)
sol[membrane₊i_Na]
```

but this does not work.

Could you help me?