Restructure and document GES code

[mhauru]

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There are a number of improvements to the GES code we would like to make, but before embarking on those, the code could use a general updating of documentation and some restructuring for easier maintainability/modifiability.

Other issues that sort of depend on this one: https://github.com/alan-turing-institute/CROP/issues/154 https://github.com/alan-turing-institute/CROP/issues/327 https://github.com/alan-turing-institute/CROP/issues/204 and indirectly https://github.com/alan-turing-institute/CROP/issues/233