text: Title: Cannot add VRT raster layer
  Body: I am trying to add a layer using a RasterData object initialized with a VRT image.
  
  ```
  vrt = RasterData("test.vrt")
  M.add_layer(vrt)
  ```
  
  The layer does not appear on the map and the Jupyter server throws this error:
  
  ```
      RuntimeError: `/test.vrt' does not exist in the file system,
      and is not recognised as a supported dataset name.
  ```
  Am I correct in assuming that GeoNotebook supports raster layers from VRTs and if so, have I correctly gone about adding the layer?