fix: exchange gain loss not set for advances linked with invoices

[nextchamp-saqib]

Problem:

• Advance Payment Entry made for a Supplier of $10000 with exchange rate Rs. 71
• An Invoice of $5000 with an exchange rate of Rs. 72 with advance entry linked is now submitted
• Payment Entry now has an Allocated Amount of $5000 in USD, & Rs. 3,60,000 in INR
• Payment Entry now has an Unallocated Amount of $5000 in USD, & Rs. 3,50,000 in INR
• Now if an invoice of $5000 with an exchange rate of Rs. 73 with advance entry linked is again submitted, it will throw debit/credit mismatch error
• This happens cuz, invoice 2 tries to allocate 5000 * 73 = Rs. 365000 while only Rs. 3,50,000 is left as an unallocated amount

Fix:

• Create an "Exchange Rate Difference" field in the advance table of the invoice
• Calculate the base currency difference due to exchange rates as follows
  • Payment Entry exchange rate = 72
  • Invoice exchange rate = 73
  • Invoice amount = $5000
  • $5000 * 73 = 3,65,000
  • $5000 * 72 = 3,60,000
  • 365000 - 360000 = 5000
• Make a gain/loss GL entry of Rs. 5000 on invoice submission to book the gain & loss at invoice date
• Fetch this difference amount & store it in the reference table of Payment Entry
• Calculate the INR allocated amount accordingly using the gain/loss difference
• The debit/credit should match as the gain/loss has been accounted for

Ref. Issue - ISS-20-21-10823

---

• Make it work for Sales Invoice Advances too
• Tests

[coveralls]

Coverage remained the same at 42.882% when pulling 7a42519 on nextchamp-saqib:purchase-exchange-gain-loss-fix into 0b320b5 on frappe:develop.