Create scaleable multi-layer map

[forman]

Users shall be able to create a scalebale multi-layer map for a specified projection from various data sources. Layers shall include

• vector data (from Shapefiles, GeoJSON, etc)
• image data generated from gridded dataset variables (from netCDF, GeoTIFF) by applying color bars
• optionally plain images (read from a JPG, PNG etc) + geo-referencing (e.g. "World file")

The maps shall optionally display legend information. It shall be possible to safe the map to an image file or PDF.

Note that displaying variable grids of netCDF files or geometries of Shapefiles will in general not require to reproject the data beforehand (see #80). This is expected to occur implicitly as only the image data and vector data (shapes) to be displayed will be remapped.

Also note that also the Desktop GUI shall later offer on-the-fly, interactive reprojection capabilities used to display spatial data on a 2D map (e.g. OpenLayers) using various projections or on a 3D globe (e.g. Cesium). As this will be achieved using web technology (HTML5, WebGL, JavaScript) it is a different feature as described above, which will likely be implemented with the help of Python libraries such as basemap or cartopy.

[JanisGailis]

The latter two bullet points we already have in the current plotting operation.