Legend for rasters

[ivanprado]

Add the possibility to add a legend to each raster.

Probably using the existing placeholder for legends that is not being used currently for rasters.

Another idea would it be to create some kind of json definition for legends that could be used by the user to define the legend similarly as it can use cartocss for defining style.

[ivanprado]

Latest carto synch introduced the ability to create custom legends. This issue should focus on enabling that feature for raster layers.

Meanwhile as workaround legends can be introduced for maps with rasters just by adding a new layer with a transparent point and creating the legend into that layer. For example: