Use TimerOUtputs.jl to measure time and memory performance

- Added TimerOutputs.jl to REQUIRE file
- JuliaFEM has now TimerOutput handle `to` and function `print_statistics()` to get summary of time and memory usage
- TimerOutputs is used in LinearSolver at the moment and tested in file `test_elasticity_2d_linear_with_surface_load.jl`

test/test_elasticity_2d_linear_with_surface_load.jl

@@ -55,3 +55,5 @@ using JuliaFEM.Testing
     u3_expected = f/E*[-nu, 1] + g/(2*E)*[-nu, 1]
     @test isapprox(u3, u3_expected)
 end
+
+print_statistics()