Tkinter based renderer fails on jupyter running remotely

Hi,

when executing the flatland rendering inside a jupyter notebook that is running on a remote host, the import of RenderRool fails with the following error message:

---------------------------------------------------------------------------
TclError                                  Traceback (most recent call last)
<ipython-input-1-942983ccfa4e> in <module>
      5 from flatland.envs.observations import TreeObsForRailEnv
      6 from flatland.envs.rail_env import RailEnv
----> 7 from flatland.utils.rendertools import RenderTool

/opt/conda/lib/python3.7/site-packages/flatland/utils/rendertools.py in <module>
      7 from recordtype import recordtype
      8 
----> 9 from flatland.utils.graphics_pil import PILGL, PILSVG
     10 
     11 

/opt/conda/lib/python3.7/site-packages/flatland/utils/graphics_pil.py in <module>
     33 
     34 
---> 35 class PILGL(GraphicsLayer):
     36     # tk.Tk() must be a singleton!
     37     # https://stackoverflow.com/questions/26097811/image-pyimage2-doesnt-exist

/opt/conda/lib/python3.7/site-packages/flatland/utils/graphics_pil.py in PILGL()
     36     # tk.Tk() must be a singleton!
     37     # https://stackoverflow.com/questions/26097811/image-pyimage2-doesnt-exist
---> 38     window = tk.Tk()
     39 
     40     def __init__(self, width, height, jupyter=False):

/opt/conda/lib/python3.7/tkinter/__init__.py in __init__(self, screenName, baseName, className, useTk, sync, use)
   2021                 baseName = baseName + ext
   2022         interactive = 0
-> 2023         self.tk = _tkinter.create(screenName, baseName, className, interactive, wantobjects, useTk, sync, use)
   2024         if useTk:
   2025             self._loadtk()

TclError: no display name and no $DISPLAY environment variable

The problem is Tkinter which depends on the presence of a local display which, in case of a server-based installation is not available.