Ernst create Layer class

Situation
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Currently we loop through the layer groups to request data, toggle or adhere to time. The layergroups loop over all their layers to do relevant things to the layers.

Problem
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The app does not have to deal with layers anymore, but the layergroups do and their is no layer class to structurally delegate this. As a result layer-group-service.js is 754 lines.

Action
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Make a class Layer and move all the layer modifications to this class.

Postponed
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- Animation logic is fully part of the layergroup.