[GNC] Precision in exchange rate conversion

[Paul Abraham]

   I have real world transaction that involves a transfer from a GBP to a
   BRL account but cannot get it to accept either the correct value in BRL
   or the actual exchange rate. It insists on truncating the rate to 3
   decimal places (or 4 sig figs?) and adjusting the amount to match, even
   though the "Transfer Funds" window suggests it's working to 6 DPs - .
   How do I fix this? Details are:
   OS: 4.19.102-1-MANJARO
   Gnucash version: Version: 3.8 (Build ID: 3.8b+(2019-12-29)) - the
   latest in the Manjaro repository.
   Amount in GBP: 1975.10
   Amount in BRL: 11102.12
   Exchange Rate: 5.62104
   Gnucash forces exchange rate to 5.621000 resulting in BRL value of
   11,102.04

   Thanks