[Post-process] Added metrics

[miguelmaso]

Added several tools for fast post-processing. Those tools are useful for generating plots after a simulation and performing tests, e.g.:

tip_displacement = Float64[]
...
function PostProcessor(...)
  push!(component_LInf(uh, z:, Ω), tip_displacement)
  ...
end
...
solve!(...)
plot(t, tip_displacement)
@test tip_displacement[end] ≈ 0.02564832