Support for discrete events accessing inputs

[gabrevaya]

Currently, there is no direct way to access inputs from within a discrete event condition. @MasonProtter suggested the following workaround. Incorporating something like this into GraphDynamics could be useful.

using GraphDynamics: SubsystemStatesView, partitioned, calculate_inputs

# To be used inside apply_discrete_event!
function get_inputs(integrator, sview::SubsystemStatesView{States}) where {States}
    (; params_partitioned, partition_plan, connection_matrices) = integrator.p
    (; u, t) = integrator
    states_partitioned = partitioned(u, partition_plan)
    i = findfirst(v -> eltype(v) <: States, states_partitioned)
    if isnothing(i)
        error("No states found.")
    end
    j = sview.idx
    calculate_inputs(Val(i), j, states_partitioned, params_partitioned, connection_matrices, t)
end