Save only final times when evaluating matter power spectrum

[hersle]

using SymBoltz, BenchmarkTools
M = ΛCDM()
pars = parameters_Planck18(M)
prob = CosmologyProblem(M, pars)
ks = 10 .^ range(0, 3, length=100)
@btime Ps = spectrum_matter(prob, ks; thread = true);
@btime Ps = spectrum_matter(prob, ks; thread = false);

Before

  351.144 ms (1483982 allocations: 274.50 MiB)
  1.090 s (1486509 allocations: 274.52 MiB)

After

  348.003 ms (1160053 allocations: 215.78 MiB)
  1.076 s (1159817 allocations: 215.76 MiB)

Not a lot faster, but a bit more memory efficient.