Disable storage of initital conditions in DiffEq solution type

By default, the DiffEq package stores the initial conditions in the solution, which is a waste of memory and may become problematic for large simulations. This can be disabled by passing `save_start=false` to the call to `solve(...)` in the elixirs (see also the DiffEq [docs](https://diffeq.sciml.ai/stable/basics/common_solver_opts/#Output-Control)).

We should consider adding `save_start=false` to the `solve(...)` call in all our example elixirs.