A more detailed revision of gncEntry and gncInvoice related rounding

First change is to ensure gncEntry rounding is consistent. Internally
calculated values in the entry are never rounded. Consumers of
gncEntry's calculated values can request them either rounded or not.

Next use a pragmatical approach for calculating values on invoices based on
the entry values: do the rounding such that we never
create an unbalanced transaction while posting
That means
- round each entry's net value before summing them in net total
- accumulate all tax totals on invoice level per tax account before rounding
  and round before before summing them in a global tax total

Hopefully this will catch a few more rounding issues in this area.

A complete solution can only offered if we allow users to manually correct
tax entries. This requires changes to user interface and data format
so that's not going to happen in gnucash 3.x.

libgnucash/engine/gncTaxTable.c

@@ -936,7 +936,7 @@ GList *gncAccountValueAdd (GList *list, Account *acc, gnc_numeric value)
         if (res->account == acc)
         {
             res->value = gnc_numeric_add (res->value, value, GNC_DENOM_AUTO,
-                                          GNC_HOW_DENOM_LCD);
+                                          GNC_HOW_DENOM_REDUCE | GNC_HOW_RND_ROUND_HALF_UP);
             return list;
         }
     }
@@ -970,7 +970,7 @@ gnc_numeric gncAccountValueTotal (GList *list)
     for ( ; list ; list = list->next)
     {
         GncAccountValue *val = list->data;
-        total = gnc_numeric_add (total, val->value, GNC_DENOM_AUTO, GNC_HOW_DENOM_LCD);
+        total = gnc_numeric_add (total, val->value, GNC_DENOM_AUTO, GNC_HOW_DENOM_REDUCE | GNC_HOW_RND_ROUND_HALF_UP);
     }
     return total;
 }