Update electricity-grid.yaml (#336)

* Update electricity-grid.yaml

add unit EUR_2020/MW to allow using this variable for extension not only of the line length but also of the line capacity

* Apply suggestions from @danielhuppmann

Co-authored-by: Daniel Huppmann <dh@dergelbesalon.at>

---------

Co-authored-by: Philip Hackstock <20710924+phackstock@users.noreply.github.com>
Co-authored-by: Daniel Huppmann <dh@dergelbesalon.at>

definitions/variable/technology/electricity-grid.yaml

@@ -5,7 +5,7 @@
     unit: km
 - Network|Electricity|Expansion Cost:
     description: Cost for increasing the line capacity
-    unit: million EUR/GW/km
+    unit: [million EUR/GW/km, EUR_2020/MW]
 - Network|Electricity|Capacity:
     description: Capacity of the line
     unit: GW