Ensure simulation.Δt is the same across MPI ranks

[simone-silvestri]

we need to add a feature that checks that

sim.Δt

is the same for all ranks upon setting it

[glwagner]

Another design is to use something like

set_Δt!(sim, new_Δt)

While overloading setproperty is nicer syntax, it is a little implicit to do an all reduce within setproperty.