Turn CISM benchmark back into a julia script

[lukem12345]

The Halfar ice model started life as a julia script on my computer. It was transformed into a .ipynb notebook to document a benchmark against the CISM results. This was then turned into a docs page to integrate with the rest of the workflow. Since we now want to execute the Decapodes-CISM benchmark automatically, this docs page should be back-ported into the shallow_ice.jl script.