Tree preparation - layers

[timlinux]

Right now we focus first on the management of layers, later we will provide better treatment to factor and dimension nodes.

For this task, traverse the tree and based on the rules defined therein generate one raster product per layer and store it in the layer folder tree as defined in #281

A quick win will be to have each layer assigned a static index value in our data model and make this option default for every layer. Then we can demonstrate tree traversal and layer generation while adding more elaborate widgets / layer generators as defined in #271

When clicking the prepare button, the following things should happen:

1. The layer is generated tree traversal starts, generating each layer output raster
2. The status indicator next to each layer should show the 'Ready for analysis' icon
3. In the layer node context menu, a 'Show layer' menu item will allow the user to add that specific raster to the map.
4. The tree model (json) should be updated to set an attribute 'Layer Generated' to true for each layer