WEU pdt-cap-ref update

The legacy value is too high, resulting in many downstream implications (e.g. higher number of ldvs produced for WEU, pkm by modes not matching to data, etc). 

The updated value is own calculation based on EU transport in figures 2022. Still margin for adjustment if needed. Full description of calucation be added under issue 558

c.f. : iiasa/message_data#558 (comment)

message_ix_models/data/transport/R12/pdt-cap-ref.csv

@@ -20,7 +20,7 @@ R12_RCPA,  4795
 R12_PAS,   6400
 R12_CHN,   8099
 R12_EEU,  13000
-R12_WEU,  18495
+R12_WEU,  15000
 R12_SAS,   3000
 R12_LAM,   5800
 R12_AFR,   2300