Fix DEIM system construction by completing system before return

[bowenszhu]

The DEIM tutorial is broken due to changes in ModelingToolkit.jl. After calling deim(), attempting to create an ODEProblem from the returned system fails.

ModelOrderReduction.jl/docs/src/tutorials/deim_FitzHugh-Nagumo.md

Lines 135 to 140 in 97aa0bc

	using ModelOrderReduction
	snapshot_simpsys = Array(sol.original_sol)
	pod_dim = deim_dim = 5
	deim_sys = deim(simp_sys, snapshot_simpsys, pod_dim)
	deim_prob = ODEProblem(deim_sys, nothing, tspan)
	deim_sol = solve(deim_prob, Tsit5())

Add a call to complete(sys) at the end of the deim() function before returning. According to the ModelingToolkit documentation, complete() marks a system as finalized and performs necessary preprocessing to register all components in the global structure. This allows subsequent operations like ODEProblem construction to correctly find all required variables and their initial conditions.