Implement crypto-to-crypto transactions processing

[cristianlivella]

At the moment all transactions have the euro as their base currency. Therefore, exchanges between cryptocurrencies are not handled.

It is necessary to implement this functionality.

For exchanges between cryptocurrencies it must be possible to choose whether:

• calculate capital gains on each trade (as currently provided for by the interpretations of the "Agenzia delle Entrate" (the Italian revenue agency);
• calculate the capital gains only when the sale is made in euro (as provided for by the bill 3131/2021).

Branch: crypto-to-crypto-trades

[Veuchez]

+1 this will be very useful! it will avoid a lot of additional work. Top!