Reorganise scripts and rules

[tomalrussell]

• workflow rules grouped by high-level concept and workflow stage
• scripts sit next to the rules that use them
• files should have been moved but are otherwise almost unchanged (apart from changes to script location references, and in one or two cases rules combined from two files into one)

Aim to improve on the sense that different sub-workflows were muddled together, as in the previous grouping by general stage (download/preprocess/etc.).

Aim for a clear set of workflow/ directories. We should be able to sketch a simple high-level dependency graph for the workflow between stages. Sets of rules within each stage are roughly linear.

For example, here's part of the high-level graph (ignoring contextual datasets):

power tropical-cyclone transport flood
  |       |               |       |
   \     /                 \     /
   power-tc           transport-flood