Modularise the way the cache is constructed

This MR 'simplifies' the logic used to map the diskcache entries to internal cache entries. This has zero functional impact now, but will make it much easier in the (near) future to move the filetype (e.g. gwf), which is currently the outermost layer, to be the innermost layer.

changed milestone to %GWDataFind Server 1.1.0

added gwdatafind_server.cache label

requested review from @duncan.meacher

assigned to @duncan.meacher

added 3 commits

• cd4dbb06...509e76a7 - 2 commits from branch computing/gwdatafind:master
• 2a8bb791 - cache: modularise the way the cache is constructed

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approved this merge request

mentioned in commit b0659447

merged