fix the tests and docstring

Author: Datseris

src/core/trajectory.jl

@@ -22,6 +22,8 @@ to the last valid point.
 * `save_idxs::AbstractVector`: Which variables to output in `X`. It can be
   any type of index that can be given to [`observe_state`](@ref).
   Defaults to `1:dimension(ds)` (all dynamic variables).
+  Note: if you mix integer and symbolic indexing be sure to initialize the array
+  as `Any` so that integers `1, 2, ...` are not converted to symbolic expressions.
 """
 function trajectory(ds::DynamicalSystem, T, u0 = initial_state(ds);
         save_idxs = nothing, t0 = initial_time(ds), kwargs...

test/mtk_integration.jl

@@ -182,8 +182,8 @@ sys = structural_simplify(sys; split = false)
 prob = ODEProblem(sys)
 ds = CoupledODEs(prob)
 
-X, tvec = trajectory(ds, 10.0; Δt = 0.1, save_idxs = [1, 2, η1])
+X, tvec = trajectory(ds, 10.0; Δt = 0.1, save_idxs = Any[1, 2, η1])
 
-@test all(diff(X[:, 1]) .> 0.001)
+@test all(abs.(diff(X[:, 1])) .> 1e-8)
 @test all(diff(X[:, 3]) .≈ 0.001)