cache the concrete_getu call

[KristofferC]

This makes the ODEProblem constructor on a large model go from

11.444860 seconds (184.50 M allocations: 6.920 GiB, 15.44% gc time)

to

10.423287 seconds (164.09 M allocations: 6.126 GiB, 13.15% gc time)