Discussion: generate aggregation tasks using modified parbake

[SGallagherMet]

Modify parbake to generate a list of file and task names and use those to create aggregation tasks in cylc.

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• Documentation has been updated to reflect change.
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[SGallagherMet]

I've pushed this branch for discussion. I had to tweak the spatial plots loader due to #1719 so this will need to be made consistent with whatever fix is applied there if it were to go ahead.

Also, not all tests are succeeding when run with make test so that should be fixed.