Preserve costs during tax adjustment (#83)

* Preserve costs during tax adjustment

Previously, tax adjustment discarded the cost of positions, causing
beancount.core.convert.convert_position to miss some conversion paths
via the cost currency. This necessitated a fallback to operating
currencies for finding viable transitive conversions.

With this patch, tax adjustment preserves the cost of positions.
Therefore, the cost currency is still available for automatic detection
of transitive conversions when computing the asset allocation.

One important assumption is that tax adjustment only ever encounters
costs after realization, as having a cost spec for the total cost would
change the cost per unit. This assumption is checked via assert and has
been manually tested without error on the multicurrency example.

The patch leaves the fallback logic for conversion via operating
currencies in place as an alternative when conversion via cost currency
fails.

* Fix lint warnings

fava_investor/modules/assetalloc_class/libassetalloc.py

@@ -6,8 +6,8 @@
 import re
 
 from beancount.core import convert
-from beancount.core import amount
 from beancount.core import inventory
+from beancount.core import position
 from beancount.core import realization
 from beancount.core.number import Decimal
 
@@ -176,8 +176,12 @@ def scale_inventory(balance, tax_adj):
         """Scale inventory by tax adjustment"""
         scaled_balance = inventory.Inventory()
         for pos in balance.get_positions():
-            scaled_pos = amount.Amount(pos.units.number * (Decimal(tax_adj / 100)), pos.units.currency)
-            scaled_balance.add_amount(scaled_pos)
+            # One important assumption is that tax adjustment only ever encounters costs after realization, as
+            # having a cost spec for the total cost would change the cost per unit.
+            assert pos.cost is None or isinstance(pos.cost, position.Cost)
+
+            scaled_pos = pos * Decimal(tax_adj / 100)
+            scaled_balance.add_position(scaled_pos)
         return scaled_balance
 
     account_open_close = accapi.get_account_open_close()
@@ -197,7 +201,6 @@ def assetalloc(accapi, config={}):
     # print(realization.compute_balance(realacc).reduce(convert.get_units))
 
     balance = realization.compute_balance(realacc)
-    vbalance = balance.reduce(convert.get_units)
-    asset_buckets = bucketize(vbalance, accapi)
+    asset_buckets = bucketize(balance, accapi)
 
     return treeify(asset_buckets, accapi), realacc