[GNC] mutual fund opening statement: ok in EUR, missing in USD

[Andrea Borgia]

Answering myself to add some information:

* when reopening the books for 2021, I see that all EUR funds had a 
split belonging to Trading:Currency:EUR, whereas this year I did not use it

* this gave me the idea to try entering the transaction from the 
"capital" account, instead of the mutual fund: this resulted in a popup 
to enter the conversion and now the capital account correctly records 
the value

Case closed.