Bugfix second yaxis

The following should be achieved:

• SMI-TimeSeries have only one y-axis (no unit is set in the respective layers, they are all the same)
  • http://localhost:3000/gdi/wis-d/de?area=3&layer=125
• show unit besides diagram when having one timeseries with one unit defined
  • http://localhost:3000/gdi/wis-d/de?area=3&layer=786
• show unit of first layer on the left (and fixed yMin and yMax if set) and all units of connected layers on the right side
  • http://localhost:3000/gdi/forest?area=9&layer=106

@rebekka.lange Please look if I have not forgotten something, what do you think?