Dirk make frontend for rain layer

x-referenced in: https://github.com/nens/lizard-nxt/pull/290

problem
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We want to eleminate the tool for rain, making rain into a layer. The code for this layer should be generic to the extend that we can easily add new layers with a temporal component (so far, rain is the only such layer).

Solution:
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- [x] Eliminate rain from the current set of tools
- [x] Rewrite map-directives.js to provide in this generic solution for layers with a temporal component
- [x] Update base.html to have no rain tool, but a rain layer