Add doc benchmarks

[kbarros]

This script will give us another way of tracking Sunny performance.

Running julia --project=@. bench.jl from the Sunny/docs directory gives this output on my Macbook Pro M1.

Cold start
01_LSWT_SU3_FeI2.jl:  40.117032 seconds (127.51 M allocations: 8.357 GiB, 7.44% gc time, 98.81% compilation time: 4% of which was recompilation)
02_LSWT_CoRh2O4.jl:   5.430768 seconds (15.11 M allocations: 2.201 GiB, 4.00% gc time, 85.55% compilation time)
03_LLD_CoRh2O4.jl:  21.163726 seconds (189.58 M allocations: 5.474 GiB, 3.26% gc time, 16.61% compilation time)
04_GSD_FeI2.jl:  34.023132 seconds (143.66 M allocations: 7.709 GiB, 1.88% gc time, 32.76% compilation time: <1% of which was recompilation)
05_MC_Ising.jl:   6.423816 seconds (628.05 k allocations: 44.459 MiB, 3.97% compilation time: 15% of which was recompilation)
06_CP2_Skyrmions.jl:  11.464052 seconds (4.04 M allocations: 268.837 MiB, 0.32% gc time, 9.36% compilation time)

Warm start
01_LSWT_SU3_FeI2.jl:   0.453626 seconds (1.54 M allocations: 322.889 MiB, 7.40% gc time, 18.17% compilation time)
02_LSWT_CoRh2O4.jl:   0.810827 seconds (4.23 M allocations: 1.514 GiB, 11.93% gc time, 4.89% compilation time)
03_LLD_CoRh2O4.jl:  17.404420 seconds (178.94 M allocations: 4.786 GiB, 1.74% gc time, 0.67% compilation time)
04_GSD_FeI2.jl:  22.972850 seconds (105.36 M allocations: 5.245 GiB, 2.78% gc time, 0.86% compilation time)
05_MC_Ising.jl:   6.179365 seconds (119.93 k allocations: 10.544 MiB, 0.26% compilation time)
06_CP2_Skyrmions.jl:  10.597724 seconds (1.09 M allocations: 67.179 MiB, 1.72% compilation time)